A gorgeous high-altitude lake in the middle of an ancient volcanic caldera, Lago de Atitlan is the perfect place to spend of week or so village-hopping between the numerous small towns around the lake's edges. Chill out, swim, relax, have a kayak, and walk in the surrounding hills (climbing a volcano if you're keen!).
The highest mountain in Central America at 4220m, this dormant stratovolcano makes an awesome climb as a side-trip from the city of Quetzaltenango. Nonprofit group Quetzaltrekkers in the town offer an awesome guided trip to the summit for sunrise (though watch out for falling rocks on the final climb to the top).