There are so many simple things:
Snorkle Ho Chan is a must, one of the best I've seen (I Scuba however).
Breakfast/lnch/dinner at Estel's, one of the most scenic down town beaches (sail boats, no commercial boats out front). Estel took quite a bit of my money on her inexpensive beer..that's alot of beer.
Hanging out at night, eating at street venders.
Driving golf cart up to the north end on the north-end plywood highway. Stop at the nice restaurant Basels? and sit up on the upper deck. Great food and relaxing. Razzing Crazy Canuck.
I recommend Scuba, remember, you can dive without certification through certain programs 1:1 with a dive master. Try it. Belize has some of the best shallow reefs.
Most over rated: Fidos.