I'd get a good guidebook, the ITMB map and maybe Emory King's Driver's Guide. I'd rent a car from Budget. I'd start with about three or four days in Northern Belize, which is happily one of least touristy parts of Belize -- Corozal Town, Lamanai, Crooked Tree. Then I'd drive to Cayo and spend four or five days at a jungle lodge, with a day trip to Tikal.
Belize books and publications by Lan Sluder:
Belize First Magazine (http://www.belizefirst.com
Fodor's Belize & Guatemala Guide
Belize First Guide to Mainland Belize
Adapter Kit: Belize (the first and only comprehensive guide to living, retiring, working and investing in Belize)
UpClose Central America
San Pedro Cool (coming soon)