The people are great, the best part of Belize. Come and have fun.
Eat the local food at the park talk to the locals.
Take a couple of tours, the tour to Lamanai is my favorite in that you get to see a lot of different things. A lot of people have an opinion on who is the best. There is more than one good one.

Take the water taxi to caye caulker for the day, hang at the split and eat at Rasta Pasta.

Bike or golf cart to explore the the island

Do a beach bar crawl, the bars are not hard to find. If you like nightlife its there to be had.

You might just go out on a snorkel tour and watch them feed the fish.