I'll cheat, and just put up some links that explain the most popular tours:
Wiki page for Cayo
Virtual Tourist
Great tours
Belize Explorer site

Pacz Tours is the most professional and qualified tour agency in Cayo. They are located at 30 Burns Ave, in the first floor of the Tropicool Hotel. They've been ranked number one for years, and are this year's winner again. Way to go Jamaal and Bob!

Actun Tunichil Muknal is the most amazing cave around Cayo. You'll say "I can't do this anywhere else" more times than you can count. This is one of the must sees of Belize! ATM wiki

Xuanantunich is a nice ruin not far from Cayo. It's great for a day trip or to take a horse ride to. Xuanantunich wiki