Visit Caye Caulker [1] and Cayo [2] (three days each).
Add Mountain Pine Ridge [3].
Add Punta Gorda [4] and the Toledo District [5].
Add a five-day side trip to Tikal in Guatemala [6].
Links:
[1] http://www.moon.com/destinations/belize/the-northern-cayes/caye-caulker
[2] http://www.moon.com/destinations/belize/cayo-district
[3] http://www.moon.com/destinations/belize/cayo-district/mountain-pine-ridge/mountain-pine-ridge-forest-reserve
[4] http://www.moon.com/destinations/belize/punta-gorda-and-the-toledo-villages/punta-gorda
[5] http://www.moon.com/destinations/belize/punta-gorda-and-the-toledo-villages
[6] http://www.moon.com/destinations/guatemala/peten/tikal-national-park/tikal