sinbi is perfect. yes its a very busy gym, but they have lots of good trainers there to take care of classes. you will always get your pad rounds in no matter how busy. they do a good job of organizing. i definitely recommend taking a few privates over the couple weeks you are there in addition to the group classes.
also, phang nga is great. my friends and i did a 3 day overnight kayaking trip and it was beautiful. highly recommended. it was the most costly single item to pay for the whole time i was in thailand, but me and my friends thought it was totally worth it because everything is included and you get a personal guide to take you to the small islands. more often than not we were the ONLY ones on the island. it was really cool. here's a few photos: