Belize is a country that should be on everyone's list of places to go.  There is a bit of something for everyone: history, culture, wildlife, adventure, trekking and other physical fun.  Figuring out what to do can be quite a task though, there is a lot of information out there - so much that it can be overwhelming.  The country is fairly large - driving from east coast to the western border can take 2-3 hours, North to South much longer.  The main highways are paved, but many areas have only rough dirt roads - so keep travel time and locations in mind when doing your planning.  Travel through Belize is pretty easy overall - they speak English, accept U.S. dollars everywhere, use U.S.-style electrical outlets and have wonderful and friendly people.  Rental cars can be expensive, but provde a bit more freedom than just going with tour guides.

Some of the major attractions that you may want to consider are:

1 - Explore the Mayan sites.  These are scattered all over the country, there are some excellent ones in and near San Ignacio Town including El Pilar, Xunantunich (Maiden of the Rock), Cahal Pech (Place of the Ticks), Caracol, and beyond the Western border, in Guatamala - Tikal.  Xunantunich and Cahal Pech have good visitor centers and you should feel comfortable seeing them without a tour guide.  On the way to San Ignacio Town from Belmopan are the Actun Tunichil Muknal (ATM) caves which are definitely worth seeing, as they are still the resting place of some Mayan artifacts - the activity can get strenuous in some parts (a little swimming and scambling up rocks).

2 - Snorkling and Diving. Belize has excellent diving and snorkling as it has the second largest barrier reef in the world.  If you want to do this you may want to visit the Cays or stay on the coast near tourist catering communities like Hopkins or Placencia.

3 - Wildlife. There are many nature reserves around the country, you can go with a tour guide or on your own.  The Belize Zoo is a good spot to visit if you don't run into the native species on your trips to the parks - it is well populated with a variety of local animals that have been rescued from various places.

Consider moving around the country a bit in your stays (rather than staying in a fixed location) to get a better feel for the country.