Another suggestion is Ambergris Caye, Belize. San Pedro is the only town there but is lively with lots of music and action. I prefer Caye Caulker though. It's smaller and therefore less lively but absolutely gorgeous and fun. Check out Lazy Lizard Split Bar.
If you don't want to go that far and you really are focused more on partying then you can always do Miami (lots of eye candy there...oops you're married...well, look don't touch). I love the art deco architecture in one little area there. I like the beaches in Ft. Lauderdale and the shopping on Los Olas.
And if you want to "rough it" in Ambergris Caye you can camp right on the beach for $7 per person which includes tent, kerosene lamp, foam mattress, and showers (10 minute walk to San Pedro):
http://ambergriscaye.com/camping/