I'll cheat, and just put up some links that explain the most popular tours:
Wiki page for Cayo
National Geographic article on Belize
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!

Easy Rider offers the best horseback riding around. Some of Charlie's horseback riding tours around Cayo include visits to such rare Mayan sites as Buena Vista del Cayo. She's the best with horses.

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

Best marine tour(yeah, this is not in Cayo, but is easily accessed from Caye Caulker) is the Hol Chan Marine Tour. Simply Awesome! The website about it is here.