There are so many thing to do on AC and other parts of Belize, if you like the water you should most defiantly do Hol Chan and shark ray alley. Awesome experience. There are several different Mayan ruin sites you can explore, but if you don't want to spend half the day getting there I suggest Al tun Ha. It is a beautiful place to walk around and explore. You also can go cave tubing, zip lining through the jungle for the day. On AC there is a butterfly jungle you can go and have lunch at and see all the different butterflies that are native to Belize. In town there are plenty of restaurants to try out, Cafe picasso is suppose to be great, the blue water grill I know has great food.
There are a multitude of bars to explore. If all those thing don't interest you there is always the beach.