Cloud forest and a steaming acidic lake--Volcán Poás Nat'l Park
Costa Rica, like most of the rest of Central America, is volcanically endowed. Climbing up to the summit of one seems to be on everyone's list--who doesn't want to stare into a steaming crater? Do this easily near San José. The Volcán Poás National Park is very accessible, even without a car. (Lots of buses will get you up there and back, either from the capital itself, or from the nearby city of Alajuela.)
You'll head up through coffee plantations on the lower slopes. At the top, (2708m /8885ft) there's plenty of cloud forest to hike in, and then the crowning glory: the mile-wide, 900ft deep crater, with steaming fumaroles surrounding the milky blue lake in its sulfuric center.
Bring a jacket--even in the tropics, it's cool up here, and rain is the rule most of the year...