Since the barrier reef is right there, you should go snorkeling. There are various tour operators all around Front Street, i think the prices for all snorkeling tours are fixed, it is just a matter of where and what you want to see. You can go snorkeling with the nursing sharks, sting rays and manatees. There are full-day and 1/2-day trips (priced about 10USD and 20USD, mask, tube and fins included). There are also lots of diving trips available.
For non-swimmers, they provide lifevests, and if you ask, the guide will snorkel with you.
This is an amazing place to snorkle. The most beautiful reefs I have ever seen. Definitely snorkle while in Belize.
Here is a close-up of a stingray, they are kind of cute. They are very mellow, they just ignore you since they are really interested in the fish the guide is feeding them with.
Here is a picture of a 'brainy' coral in Caye Caulker... I saw lots of damaged corals, i think mostly in part because of anchors thrown in the water and then pulled... such a pity...
Here is a whole family of stingrays. Their mouth and gills are located on the bottom, so they kind of 'sit' on top of the food in order to eat it..