If you want to do something crazy, fun, memorable and not invest too much money in it, here is what you have to do:
Wherever you are in Thailand, rent a bike and go crazy. It will cost you around £6 / day plus gas which is another £2 and you will have loads of fun. We did the same, instead of taking a shuttle or taxi to the Elephant Sanctuary in Phuket we rented a bike and went up and down the mountain with it. High adrenaline for sure, scary as hell driving in the most crowded place on earth but surely loads of fun.
Check out the video, see the places we visited with the bike. You can see on my face that I had several moment when I was really really scared, but maan, these are some great memories.
The video is quit long as it is almost the full length of the journey, if this is your thing, enjoy a goo ride.