If you are into Diving Belize is famous for the Blue Hole - a stunning sink hole in the ocean that has giant stalactites. It is a deep dive (40 meters) although they tend to let people on Open Water Cert go anyway.
If you don't have your scuba licence the snorkeling off Caye Caulker is the best we have done in terms of wildlife. Here is some of what we saw: http://fivepointfive.org/photo-blog-photo-of-shark-snorkel-caye-caulker/
The last bit with the sting rays and sharks was sooo much fun! We decided to stay on Caye Caulker because it is much quieter than the larger islands, is is a gorgeous sleepy island made of white sand. about 7 years ago
Boat tour up the Monkey River near Placencia is unbelievable! We saw a whole family of monkeys, boas, crocodiles, tarantulas, manatees in the jungles and/or water. Had lunch with the boat guides family.
We also cave tubed. Lots of fun and the water is refreshingly cool.
Snorkeled off of Laughing Bird Caye. Swam with mantas, turtles, and barracudas.
Our base of stay was outside of Placencia. about 7 years ago
If you are driving, I would suggest Chaa Creek area (Macal River), cave tubing down Caves Branch River, Placencia area if you want a real beach, San Pedro (Ambergris Cay) for great snorkeling/diving/fishing and if you can pull it off, at least one night on an island like Long Cay (Glover's Reef Atoll) http://www.offthewallbelize.com/ to really feel like you have gotten away. US dollars spend well everywhere. about 5 years ago