It would depend a lot on where you're staying, but personally I like Bali - it's different. When first arriving to Asia it took a couple of days to get used to it, the same thing happened with Bali, you need a few days to adjust to it. There is lots to like about Bali (friendly people, lots of greenery, relaxing atmosphere, good weather, relatively cheap) but the best part is that you have to put real effort if you want to be stressed out. For some reason "it will pass" thinking just invades your mind and allows you to relax.
I'm still quite torn about Bali, it's a great place to spend your time, but at the same time in here I have to deal with suicidal thoughts way more often than I would prefer. The saying of "It's not you, it's me" would fit great in this case. So it's very hard to decide if I ever want to go back here, I don't want to spoil it for myself in case I will finally grow up and be able to enjoy everything in here. In any case a visit to it made me a better person (or at least showed how to work towards being one). Still, if someone would ask if they should visit Bali my answer would be yes, it's just special.
Something I really liked about Bali is geckos. They just always make me smile or laugh or just bring out silliness in me. Big or small, I'm always glad sharing my space with them (maybe not so much in the middle of the night when already having some screwed up dreams and they decide to say hello).
Something I did not like is all the smoking. I try to avoid it as much as possible, and can just avoid it once in a while, but in Indonesia where was way too much occurrences where I had to deal with smoking. Addictions are for life, and it feel like it was being poked too often. Think I would have left Bali just after two months to just avoid the temptation of lighting up a cigarette.
Some short summary about different cities. Sanur small town by the beach, good place to start the trip (and where is islands nearby), but unless you're of retired age it might be too quiet. Beach not too clean. Ubud is my favorite town (at time or writing) in Bali surrounded by jungles and all the nature. Very chilled out atmosphere and provides lots of opportunities for meditations / yoga / workshops. Canggu surfing town by the beach. Waves seem a bit dangerous if you trying it for first time (maybe first try Kuta). Lots of places to eat (organic, paleo, etc). At the same time lots of dogs that get aggressive after sunset. Beach not very clean, but feels nice walking in ankle deep water. Has different vibe, but still feels like Bali. Seminyak is way too dirty and touristy to be enjoyable. If I didn't seen some sculpture once in a while I would forget that I'm in Bali right now.
Watch out for Nyepi as during it no shops are working, internet is down, and there is a lot of other things which might make your trip less enjoyable (or prepare for it by getting snacks, movies, etc).
The currency is Indonesian rupiah (IDR). The rate is about 16600 IDR for 1 EUR (or just throw out last 3 numbers and divide by 16.6), 100K IDR for about 6 €, 300K IDR for about 18 €. You can sometimes get coins, which I'm not sure that you should be doing about.
Some places accept payments by card (might add a transaction fee), but most of transactions happen in cash. Watch out when using the card, because internet is full of horror stories of their card being skimmed. Some people suggest to disable ATM withdrawals (it also seems that ATMs use swipe payments) on your card until you need to use, and even replacing your card as soon as you leave Indonesia. There is a limit of how much you can withdraw from ATM per day (1.25M to 2.5M), could still use your card at other ATMs. Try to avoid ATMs in shops (as owner might be the one who is skimming), use those near the banks and being recorded by camera.
If you use cash exchanges don't use the first one you see, walk around until you have at least multiple places to compare the price (there is a lot of them around and prices can have a noticeable difference).
This information will be different depending on your nationality and even if it's the same it might be outdated, check to be sure. I like to check this Wikipedia page which lists all requirements for Lithuanians and VisaHQ to be sure. You can also Google using your native language which might provide some useful articles.
You can get visa exemption after landing and it will be valid for 30 days, you can't extend it and you need to leave before it ends (could stay a few days longer by paying a fee). Another option is getting visa on arrival (VOA), which costs 31 € (can be paid in multiple currencies), and can be extended by another 30 days. You could do the extension yourself, but it will require going to immigration multiple times and unless you're really strapped for cash it's just not worth it. You can use a service like Visa4Bali who for about 50 € will take care of most of the stuff for you (pick up your passport, fill all the documents, deliver passport to you, etc). You will still need to go to immigration to take your fingerprints and photo (which takes about 2 minutes, but driving between cities can take hours).
You can get a local SIM card from Telkomsel. Some people are suggesting to avoid getting a SIM card in the airport as you will get scammed, but you might as well get scammed in town (prices ranging from 80K IDR to 400K IDR for the same thing). The speeds are OK when it's working, but you reach the points with no connection. Data packages are also a bit of the scam, because if you buy 17 GB package for example it's a combination of (normal data, local data, chat data, video data, etc), so if you mostly use international websites it might run out much faster. You can top up with just the internet package, for 3 GB (valid for 7 days) it's about 34K IDR (~2 €). It also seems that recent change requires the SIM card to be registered (which sellers seem to be starting to do now on purchase).
There is a lot of places to get food, walk around and choose something that you think might have something good. If you're really lazy to go out you can always try using GoJek food app for delivery. Keep in mind that most of the time government and service taxes are not included in the menu.
Warung Makan Little Mars is a small family run restaurant in Sarun and it feels very cozy there. Light background music, everyone is chatty/friendly, and it has doors so you don't get much noise/pollution from the street. Food is tasty and prices don't bite.
Cafe Wayan and Bakery restaurant with great food and decent prices. Have all kinds of food (western, indonesian, etc). After finding was my go to place for almost every day.
Crate Cafe restaurant in Canggu, good for breakfast. Lots of options (can start with Brekke plate). Can get busy.
Taco Casa Canggu restaurant in Canggu (also available in other cities), big portions, decent prices. Could be good choice for slow carb diet (by removing some ingredients).
Hubud is a co-working space in the Ubud. Normally I try to avoid co-working spaces and rather spend that money to get fancier Airbnb which would be comfortable to work from, but if you're staying nearby it's worth to get at least the 30 hours membership (around 55 €) for the events (something is happening almost everyday), for meeting new people, and when internet goes down at your Airbnb.
Tokei Ubud Climbing Gym a bit daunting for a first time, but after you get familiar with it a very good way to get some exercise into you.
If you plan to do surfing and it's your first time probably (have not done myself) start at Kuta which seems much safer. The waves at Canggu can get rough.
Walking around you will constantly be asked if you want a massage. Treat yourself to them. You could get a massage for around 6 € from cheaper SPAs. If you want to get something more fancier (around 20 € for massage) go to Jaens Spa Ubud or Fresh! Spa.
There is a lot of yoga / meditation places.
If you need a doctor most cities (bigger ones at least) have 24 hour doctors, just Google for one. Price for consultation (in Ubud) was 400K IDR (~ 25 €).
Traffic / Walking
It depends on where you are and where you're trying to get to, but most of the time you can find sidewalks or walk by the side of the road. Traffic in Bali is crazy, so watch out, but it's possible to walk around. During the day it can be very hot and that time is better spent somewhere indoors with AC working.
Sidewalks often are not the best ones so you need to watch where you go. They like to go up and down, have holes or open sewers, be occupied by dogs/monkeys/scooters or people (they are only wide enough for 1 or 2 people). Especially watch your step during the night as most of the streets are only lit by nearby shops or passing scooters. Still, if you watch yourself it is possible to reach a lot of places just by walking.
Some cities don't like when any of the transport apps or taxis are used (to the point where it makes you think the locals will drag everyone out of the car and beat them). It's best to ask the host of the place you're staying if they have a driver they would recommend, it has a lower chance of getting scammed. For 400K-800K IDR (25-50 €) you could get a driver for half or whole day. Blue Bird Taxi is the official taxi, but everyone else copies them to the point where even knowing what to look for it can be hard to notice which one you're using.
If you can drive a scooter it will provide with much more freedom. Of course traffic is crazy so it might look like the last thing you might want to do (this is even for people who drive motorcycles back home). Even if you can easily rent a scooter you need to have the international driver's license to drive legally (bribes for police are relatively small, but if you're driving illegally and get into accident the insurance will tell that it's your problem).
Entertainment / Visit
Electric bike tour around Ubud. Great choice if you want to see all the rice terraces and surrounding area. It was really fun, my ass hurt a lot after all the driving, and I'm still surprised I'm alive (a few tricky corners which might lead to ending in the ditch or a decoration of an incoming car). The electric part is convenient as you can do most of it without needing to cycle (except for tricky corners and hills). Suggest to do the morning one as towards the end of the tour it was way too hot (even when being able to get some speed from the wind).
Quad bike ride in jungle was fun, but at the same time very dangerous. I like doing something extreme, and even fine about getting hurt during them, but this made me think if risk/reward ratio is a good one to continue it. It happened 5 minutes into a ride when going down the hill into the jungle two girls riding in tandem drive off the road through the border and then through the guide.
TEDxUbud is a great event if you get a chance to attend it. Lots of great talks, chances to meet new people, and well organized.
FuckUp Nights is another great event you should attend in Ubud (if it's happening during your stay). Lots of great talks and chances to learn something new or meet new people.
Bali Coffee Tasting, Brewing & Latte art great workshop especially if you like the coffee (I don't). Provides the theory about what you will be doing and when you can do it yourself to get the practice and see the difference.
(from external source) Daily in 30s, nightly in 20s. Wet season January and February (it rains hard).
I was visiting during April and May. Weather is nice, it gets hot during the day (great time to do some work or jump in the pool), but around 5 PM it starts getting pleasant for walking around. During the night it can get cold, and I woke up a few times to burrow myself under the covers. It rains sometimes, hard, but did not bother me much as it often happened when I was not planning to go anywhere. It's relaxing to listed to rain, and it does not last long usually.
For other locations in the world see intro article.