In San Juan, baking on Puerto Rico's sun-saturated beaches has become a sport unto itself: Assume prone position, hold for 30 minutes, roll, repeat. Yet adventurers don't have to sleepwalk through their island vacation: Mountains, parks, rain forests, bays, and caves—all near the capital—provide the perfect adrenaline fix.

"People see Puerto Rico as this tiny island with casinos, beaches and a coast," says Raymond Sepulveda, co-founder of Acampa Nature Adventure Tours, an outfitter based in San Juan. "But you can go to five ecosystems all offering different activities. You will never be bored."

Need proof? Here's a (small) sampling:

* Run, bike, or hike along trails that ribbon through lush forests, an orchid garden, a lotus lagoon, and other botanical delights blooming in the University of Puerto Rico's Jardin Botánico, 45 minutes from Old San Juan. "It's a good place to get out inside...the city and just feel free," Sepulveda says.

* Kayak or windsurf in the calm La Laguna del Condado, a saltwater channel streaming through the upscale tourist district of San Juan's Condado Beach.

* At Piñones Forest Reserve, about 30 minutes from Old San Juan, you can mountain bike, hike, or bird watch in the island's largest mangrove forest and along the park's soft sandy beaches.

* Near Bayamon (also known as Pork Rind Town) in the San Juan metro area, the Parque Ecuestre Julio Enrique Monagas attracts horseback riders, mountain bikers, hikers, and climbers who ascend limestone rocks with a 5.7- to 5.12-degree of difficulty.

* Surfers, windsurfers, and kiteboarders ride the swells off Jobos Beach, about an hour west of the capital. The coral reef adds an element of danger.