If you can't dive before you go, you must learn. Belize has some of the most outstanding dive sites in the world, and the barrier reef is spectacular. It is second in length only to the Great Barrier Reef of Australia, and despite having a lower diversity of fish species it more than compensates with abundance and richness of corals. The cayes are an excellent location to stpo off and recharge you batteries if you are on a long trip, with crystal seas, golden sand and the bluest sky.
Visit Belize Zoo. Even if zoos aren't your thing I promise that you will enjoy the Belize Zoo and Tropical Education Centre, and will have the opportunity to see some of the amazing wildlife of the country at first hand. Animals are often just out of sight in the rainforest, and can only be heard through the dense vegetation or require good eyesight!
Belize in one of the countries that was home to the ancient Mayan civilization, and there are a number of archaeological sites throughout the country. If time in the country is restricted and you can only visit one site go to Lamanai (the submerged crocodile) reached on a cruise along the New River Lagoon through the rainforest from the town of Orange Walk in the north.
There are a number of parks and reserves throughout the country, in fact around 40% of the country is protected for the conservation of biodiversity (the rich range of plants and animals). Many of these are run by directly by the local communities, providing an income for people who would otherwise survive by clearing the rainforest for farming. Give your support to conservation, the environment and the communities of Belize by visiting them. Part of the departure tax paid at the border or airport on leaving the country is used to support the conservation work undertaken in Belize.
Quick Tips:
Belize City has a bit of a reputation for crime, especially if you are white and/or a woman. Don't walk around at night - take a taxi. Compared to other countries I have visited there isn't so much hassle though, and once you have gained a tan (not pink sunburn!) they don't consider you as interesting as a new arrival and will leave you alone.
Best Way To Get Around:
Taking the bus can be quite an experience, as you are likely to share your journey with three times the recommended number of passengers, some chickens, the weekly rations for a small village and a few jerry cans of petrol. The journey will be long, as the roads are mainly potholed dirt tracks, especially when you reach the south of the country. Crossing bridges can also be a little hair-raising, as these tend to be single track and in a dubious state of repair. Most driving is on the left (as in the UK), but the buses are usually American, so you get off into oncoming traffic.
To reach the Cayes, take a water-taxi from the waterfront in Belize City. Then sit back and hold on tight for a speedboat journey to your destination. You will probably get wet, but its the most exciting way to arrive in the cays.