Reviving this post, just to add some helpful hints ^_^
Chiang Mai - small-ish city, most bars/clubs/restaurants close by midnight but there are places that open later (you just have to look for it), great area for hiking, trekking and in general, doing all of the adventurous stuff. It's just a very chill laidback vibe in comparison to Bangkok. If you're going here to go see elephants, please do not go on those elephant tours advertised everywhere. They most likely mistreat the elephants. But there is an elephant sanctuary nearby that you can go to where you not only learn about the elephants, you also get to ride them bareback. Food-wise, you have to try Sai Ua (Northern Thai herbed sausage) if you're there. It's quite spicy and especially if you eat it with nam prik, but Chiang Mai is known for it.
Phuket - your typical beach tourist area. It kind of feels like Magaluf because there are so many tourists and everywhere caters to it. There's even a street full of bars that feel even more like it. In terms of the beach though, it's not the most beautiful, especially in recent years. Honestly, if you're flying to Thailand and want to go to the beach, this really isn't the place to stay. It's good for a weekend and if you want to drink/party. And if you're rich as hell, you can go to the other side of the island where all the 5 star resorts/spas are. Now, that area is super nice.
There's also Chiang Rai that people should visit but I haven't been yet. Or maybe I have and was too young to remember. Anyway, it's also one of the places to go see outside of Bangkok and the Islands.
Bangkok - look, a lot of people dislike it because it's huge, lots of traffic, lots of pollution and very urban, but it's home to millions of people and it has its great aspects you just have to look. It really is one of the fastest developing cities in Southeast Asia. There are lots of places to go see as a tourist like the Grand Palace, Chatuchak Weekend Market (you can literally find everything here) and the Floating Markets. Lots of large department stores with restaurants and cafes and dessert places. Ooh and MBK is the mall you want if you're looking to buy cheap stuff. Khao Sarn Road is all the area most tourists stay in because it's cheap but honestly, this is such a party street like stay here at your own risk because it will be loud. Lots of bars and clubs around there that stay open all night. Oh and from Bangkok, you can go to Ayutthaya which is the old capital city, so that is certainly worth seeing.
Other areas worth seeing that aren't as popular are: Khao Yai, Kanchanaburi and Cha Am.
The Islands:
Koh Phi Phi - Near Phuket, famous for its Half-Moon Party, and small tiny island. It really is a party island tbh, like there's a booze cruise that starts here that's pretty popular. But the best thing about Koh Phi Phi is taking the full day tour of the surrounding smaller islands because you get to visit Maya Bay (where the movie The Beach was filmed), monkey island (harmless and adorable!) and shark point (they're just reef sharks and you can go swimming with them!). You can go snorkeling and scuba diving around here as well.
Koh Lanta - there isn't much to do around here except visit Old Town for the restaurants and the mangrove forest down south of the island, but it is such a chill, relaxing place. Perfect to just lie back and forget the world. The beaches aren't great because this area has lots of rocks. There's a cool strip of beach area for bar stalls. Not a party island like Koh Phi Phi but still fun. You can also do another boat tour here where you can go snorkeling. I haven't done all the activities here yet because I mostly went there to lie in the sun and relax lol
Railay Beach - actually my favourite place in the world. It's probably the most beautiful place in Thailand, imo. The beaches are gorgeous, like this is what you'd imagine when you think of Thai beaches. It's a small island and it's famous for rock climbing, so if you're into that, this is definitely the place to go. You can also hike up this mountain for this stunning view of the island, but then if you go further inland, you get to a lagoon with mountains surrounding you on either side. You have to go during high tide so the water's high enough. There are also paddle boarding and canoeing (although you can find that on most islands) but you can go paddle boarding at night and see luminescent fish, etc. Super cool! Not really a party area, mostly just chill bars, but there is one called End of the World Bar (i think?) where you can have a dance.
Koh Samui - I haven't been back in awhile, but there's a lot to do here and it's the main island that people fly into and ferry out to other islands like Koh Tao and Koh Phangan. And while you can walk from one end to the other on Koh Phi Phi and Railay Beach, you cannot with Koh Samui. It's a very large island, but the main tourist area is all in one place. I honestly didn't do any of the touristy stuff here last times I went, so I don't quite remember what there is to do. Ooh, there's a restaurant called Coyote's that makes great margaritas (if it's still there).
Koh Phangan - this is The Party island where the Full Moon Party is held. Like that's all this island is famous for and all it really is there for. I don't think I've ever stayed here because most people will just ferry in for the party then stay up to ferry out at first light. Tips for the Full Moon Party: do not lose your friends, stay together, be very wary of the buckets because there is a lot of alcohol in that, like more than you realise. Ummm... don't wander off, stay in the lit up areas. Although I think it's just a hyped up party, for foreigners, I suppose it's really awesome and super fun lol.
Koh Tao - if you're huge into scuba diving, this is the place to do it at! I don't have my license yet so I can't really tell you much about it, but according to my friends, this really has some of the greatest spots to go scuba diving. I haven't been back in a long time but I really loved how cute and small it was (or at least the area I stayed in).
Similian Islands - again, this is probably one of the best places to go scuba diving as well. I've never been yet because I have yet to get my license, but I'm definitely going there as soon as I do!
That's all I can remember so good luck to anyone going! ^_^