If you're seriously interested in the Maya, do not miss a trip to Caracol with a qualified guide. Nearby Tikal in Guatemala is awesome indeed, but Caracol is still being actively reclaimed from the jungle with frequent new discoveries: a fascinating view into archaeology at work. Comparing the two old rival city-states was very educational for me.
There are other very important Mayan sites in Belize including Santa Rita (Chetumal), Cerros, Lamanai, Nohmul, Cuello, Altun Ha, Xunantunich, Pacbitun, Pilar, Maintzunun, Lubaantun, Nim Li Punit and Uxbenka.
When you're full-up with archaeological stuff, head for Ambergris Cay and relax with a tropical drink and listen to the surf roaring against the second largest barrier reef in the world. Or put on your swim/snorkeling/diving gear and enjoy swimming with the rays and nurse sharks and other adventures.
How safe is it for Americans to travel there now? I’d love to do this. How is the dollar doing versus Belizean currency?
Is it now safe to travel to Piedras Negras?