I'm making my 3rd trip to Belize this March. I LOVE IT!!! My favorite town is Placencia, small fishing village on the peninsula. Some cool activities are Xunantunich ruins, Rio Blanco Falls, Laughing Bird Caye, Cockscomb Basin Jaguar Reserve. You can book all these activities with Caribbean Travel & Tours in Placenica... here is their link
http://www.ctbelize.com/placencia-hotels.htmlI especially like Xunantunich. It takes a good 3 hours to get to from Placencia and it's a beautiful drive. If you want to do something unusual.... try a night time hike through Cockscomb Basin to look for Jaguars!!! If you do book any of these tours through Caribbean Travel, request Basilio as your tour guide, he's super knowlegable and very funny! Just watch out if you go on a jungle excursion with him... he'll make you eat termites!!! They taste like mint.
Also, the last local flight (Tropic Air or Maya Island Air) to anywhere out of Belize City leaves at 5pm... so if you get there after that, and don't want to stay in Belize City, try Black Orchid Resort in Burrell Boom, it's really nice and they pick you up from the airport.