On our third day in Belize, Caracol was the destination. Caracol is a very large (and very out of the way) Mayan site up in the foothills of the mountains in the Cayo district (west-central Belize). It's seriously remote--bring lots of water, and make sure there's lots of gas in your truck! It took us about two hours of driving on some very, very bad roads to get there.
Obviously the trip was worth it. We got there around 9 or 10 am, before the tour groups arrived, and before the sun got too oppressive. There was a nice mix of excavated ruins in the sun, and unexcavated or partially-excavated ruins beneath the jungle canopy.
... and sports a pretty awesome view from the top. Never mind the dude with the machine gun.
A lot of the structures have cool rooms you can go inside.