soz for late reply.
On Koh Phi Phi we rented kayaks which was good fun and the water is so clear. we paddled round to 'Monkey Beach' which is an experience in itself , those monkeys are vicious! Koh Phi Phi is quite touristy to be honest.
We Also went to Koh Lanta which is stunning but very quiet. Had my first experience on a moped which was fun but im not too good at it and nearly came off it a few times so not in a hurry to try again!
Koh Samet is a great little island , you get the bus from bangkok then a ferry. Its totally laid back with cafes / restaurants on a stunning beach and soe bars too. felt like it had the balance right. Coud have stayed there alot longer tbh.
We didnt actually do THAT much as we had just over 3 weeks and we wanted to relax rather than run around trying to see loads of places