Hey EnanitoVerde,
The Blue Hole is a beautiful dive, usually full with sharks! You need to be an advanced diver for this dive however and your last dive should not be more than 6 months ago. I recommend you visit Caye Caulker and arrange the dive from there, or do a snorkel tour from there. They offer many great tours. I really like the shark and ray alley, you snorkel with large, free groups of stingrays!
When visiting Belize, you should not miss out on the caves! The ATM cave is amazing and you can find information here:
www.onejungle.com/all-our-tours/one-day-tour-belize/atm-cave-tour
Cave or river tubing is also a lot of fun..
Also, if you are interested in culture, you should mingle and learn about the Garifuna. They have a very interesting history and there are few of them left. You can find them all along the Caribbean coast of Belize south from Dangriga. Hopkins and Punta Gorda are good places to mingle with the Garifuna.
Enjoy Belize, you will LOVE it!