The Ruins of Copán [1]: This wonderful Mayan city just across the border in Honduras is home to some of the finest carved stelae in the Mayan world and to an outstanding museum housing several original monuments.
The Ruins of Quiriguá [2]: Harboring beautifully carved stelae set amid luxuriant banana plantations and jungle, this small Mayan site is one of only three UNESCO World Heritage Sites in Guatemala.
Cerro San Gil and Río Las Escobas [3]: This fantastic rainforest preserve harboring a protected watershed with waterfalls and pristine pools is just minutes away from the cruise-ship terminal at Puerto Santo Tomás de Castilla.
Punta de Manabique Wildlife Refuge [4]: A remote peninsula abounding in wildlife and pristine nature, it also offers deserted beaches and the opportunity to explore adjoining wetland canals for excellent bird-watching.
The Zapotillo Cayes [5]: Sail or motorboat to the picture-postcard beaches of this remote island chain on the southern fringes of the Belize Barrier Reef for scuba diving, swimming, or just plain relaxation.
Bocas del Polochic Wildlife Refuge [6]: An excellent spot for bird-watching, this seldom-visited park has excellent visitor facilities from where you can explore jungle trails, wetland canals, caves, and sinkholes.
Links:
[1] http://www.moon.com/destinations/guatemala/el-oriente-and-izabal/copan-archaeological-site/copan
[2] http://www.moon.com/destinations/guatemala/el-oriente-and-izabal/quirigua-puerto-barrios/quirigua
[3] http://www.moon.com/destinations/guatemala/el-oriente-and-izabal/puerto-barrios/puerto-santo-tomas-de-castilla/cerro-san-gil-and-rio-las-escobas
[4] http://www.moon.com/destinations/guatemala/el-oriente-and-izabal/puerto-barrios/punta-de-manabique-wildlife-refuge
[5] http://www.moon.com/destinations/guatemala/el-oriente-and-izabal/livingston/near-livingston/the-zapotillo-cayes
[6] http://www.moon.com/destinations/guatemala/el-oriente-and-izabal/lake-izabal/near-el-estor/bocas-del-polochic-wildlife-refuge