Hmmmn! We just had a group of 5 adults from Minnesota stayed two nights and chose TIKAL for one day and the Actun Tunichil Mucknal cave trip for the other. They had flown into Cozumel and bused down to Corozal, spent overnight and took the water taxi to San Pedro and promptly took another water taxi to Caye Caulker. I picked them up in town on the coast a hundred miles away when they came in from Caye Caulker. From me in Western Belize, I delivered them to the port again and they were going to do Ambergris Caye before returning the same way back through Mexico to Cozumel. In Belize, they were doing different adventures every day on a 7 day trip.
Depends on how tight your time is? Here is a website which lists SEVEN hostels around Belize for low budget sightseeing and adventuring. http://folkmusicfl.tripod.com/hostelsandhotelsinbelize
There are too many attractions and adventures to complete in less than maybe eleven months. You would really have to be choosy with your time and interests.
Falconview Tourist Backpackers Adventure Hostel