Dive with the Sharks
If you're 12 years or older, and you can swim alright then you can jump in with the sharks! You need to book in advance but diving sessions run several times a day, every day of the week. If you have never dived before then you need an introductory session first of all, but they also offer 'tune-up' sessions for those who perhaps haven't dived in a while, or are not confident divers. You need to arrive fifteen minutes before your dive is scheduled, which is important because you can't get refunded for sessions that you miss. Just bring your swimsuit or trunks and away you go! If you have your own diving mask then you're welcome to bring it but all equipment will be provided.
After the session you can take home a certificate of your diving experience - memories really do make the best souvenirs.
Prices (as of March 2013) are;
Certified Program - Baht 5,300
Scuba Tune Up - Baht 6,000
Introductory Program - Baht 6,600