Snorkelling/ Diving
Thailand has really accessible marine life that you can see a stone throws away from the shore so there's no excuse not to dunk your head in the water. Koh Tao offer great diving opportunities if you're daring enough or simply go for a little snorkel off one of the islands. And don't worry if you can't swim, they give you a little life jacket so you can bob around while looking at the fishes.
Full Moon Party
No matter how old you are, if you haven't been, just stick your head in - you won't have seen anything like this. Fire and neon are absolutely everywhere! If you don't like parties just go to say you've done it and then you can moan about it as much as you like. If you do like parties, you'll have the best time but just be warmed the buckets are stronger than you think and you wont have had a hangover like it. So worth it though!
Take a trip on a long-tail boat
Thailands Tuk Tuk for water. You'll have plenty of opportunities as they're everywhere but definitely a must do. Inland on the lakes nothing quite clears your head like it. On the sea as you can imagine the journey's a bit messier but they still provide access to places you wouldn't see otherwise.
See flora and fauna in one of the national parks
There are lots of national parks dotted around the country where you can see all sorts of weird and wonderful plants and animals. You'll likely see Macaques, you may see Elephants and if you've taken a dose of liquid luck you may even see Tigers. That last one is very unlikely but it's fun to look all the same.
Visit the Elephants
There are lots of Elephant sanctuaries all around the country, where you can splash around the water with them, give them a good wash and feed them sugar cane. Before I went I researched extensively places where you could ride Elephants that looked after them properly. The first place seemed good, but how do you really know! A year later I did it again and saw a more run down, less ethical version. To keep them happy and looked after I think skip the riding and give them a good mud bath instead. I know I'd prefer that!
Enjoy a bucket or two on Khao San Road
Crazy Khao San Road will likely be one of the first places you visit on your trip to Thailand. Situated in the old town in Bangkok, it's traveller central with bars, street food and pop up market stalls selling cheap clothes. It gets super busy but it is a good way to get you used to the chaos; generally places will seem milder after this. You can fill your bucket with your spirit and mixer of choice but go local with Sangsom rum and an energy drink called M-150.
Stroll along the street markets
There are so many wonderful markets throughout Thailand of both the day and night variety. Skip your restaurant dinner and eat your way from stall to stall, buy some traveller essentials such as snake charmer pants or buy some tat that will amuse you for all of five seconds. The wooden frogs that when stroked with a wooden stick make a vague ribbit sound are a great investment!
Get a Thai massage
You have two choices when getting a massage in Thailand. Either venture in to a dodgy looking parlour and hope it is as you think or get one in the middle of a public place for all to see. Personally I prefer the public option. Then at least you know exactly what you're getting. They are cheap but shop around if you want a traditional thai massage because a lot of places provide a glorified back rub.