San Pedro (Ambergris Caye) is great. Probably has grown a good deal since I last visited 11 years ago. You can take a 1.5 hour water taxi, or take a quick 15 min flight. Plenty of fun stuff to do on the island, including snorkeling.

If you are interested in ruins, Xunatunitch is really amazing. It was in one of my areas, so I saw it often. It's up near San Ignacio and the boarder with Guatemala.

I didn't get to go cave tubing, but that's a common tourist thing that people love. Can't really go wrong. Belize is great.