One of the most important of all Maya civilizations lived, prospered, then mysteriously crumbled around the Copán archaeological ruins, a Unesco World Heritage Site. During the Classic period (AD 250–900), the city at Copán Ruinas culturally dominated the region. The architecture is not as grand as that across the border in Tikal, but the city produced remarkable sculptures and hieroglyphics, and these days you'll often be virtually alone at the site, which makes it all the more haunting.
The ruins are a pleasant 1km stroll outside of Copán. A visitors center, an excellent sculpture museum and a cafe and gift shop are close to the main entrance. The guides at the Asociación de Guías Copán really know their stuff and hiring one is a worthwhile investment.