Xunantunich
Close to Guatemala you can stay in the Cayo district and do this Mayan site. For more detail visit http://en.wikipedia.org/wiki/Xunantunich
Caracol
Again in the Cayo region the largest and still not fully uncovered of Mayan ruins in Belize. The main largest temple or structure is Canna at 143 feet high and that after the site being on a plateu.
Tikal (Guatemala)
One of the more famous Mayan ruins not in Belize but accesible via Belize is Tikal. You can get overnight trips from the jungle lodges in Cayo. It was one of the more important Mayan cities and it takes a full day to appreciate it's excavations.
Lamanai
Located in the north of Belize in Orange Walk is Lamanai meaning submerged Crocodile. It has a spectacular location on a lagoon. You can get access and stay at Lamanai Outpost Lodge.
Lubaantun - Nim Li Punit
Acces via Punta Gorda or Placencia if you may be in the South of Belize - Toledo district. The largest recovered stelae in Belize was found here in this thought to be ceremonial ancient city. Ball courts and Plazas and the ever present howler monkeys too !