Of course, you'd be labeled soft on adventure if you skipped Caribbean National Forest, also known as El Yunque, the 28,000-acre U.S. national forest 30 miles southeast of San Juan. Visitors can hike deep into the country's only tropical rainforest along 24 miles of trails fragrant with sweet frangipani and humming with the stereophonic chirps of Coqui tree frogs.

Those who seek a bit more speed and roar can hop on an ATV and swashbuckle through the rain forest's "roots." Since El Yunque is protected, the all-terrain vehicles are confined to the base—but there's plenty of starter rain forest to explore and bumpy terrain to tackle in the country surrounding the national forestland.

"It isn't hard to get the ATVs up there," guide Carlos Garces informs a group of seven as they straddle their big wheels like oversized children. "But it's hard to get an ambulance there."

The trip, organized by Hacienda Carabali, covers ten miles of rugged trails that zigzag through the foothills, cutting between private and public land, from a dairy farm to a valley covered in green shag carpeting to a set being built for a Hollywood movie shoot. The rock-strewn path has some quick switchbacks, pot-hole plunges, and rivulet crossings that drench pant cuffs. But you won't need a life insurance policy and a prayer to survive; a firm grip and fast reflexes will suffice.

El Yunque is the island's only rain forest, yet there are dozens of other parks (wet and dry) to tramp, swing, and swim through. For instance, in the eastern portion of the Karst region, a northern band of limestone formations between Aguadilla and Loíza, seriously athletic visitors can test their endurance on Acampa's half-day, high-cardio excursion, the Organic Tanama River and Cave Adventure. Activities include hiking, a river dip, cave exploration, beginner spelunking, rappelling, a zip line, and lunch—but not in that order.

The outing starts with a moderate hike through untamed forest still moist from the last rain. The guides stop in fits and starts to share their eco-wisdom: Eat three red fruits from the acerola tree, for example, and you will get your daily dose of Vitamin C.

After hiking for a solid hour, the guides lead the group into a voluminous bat cave where the tiny winged creature hang from the ceiling in posses, their shrill chorus reverberating off the stalagmites, stalactites, and petroglyphs carved by the indigenous Taino. A crawl through a more claustrophobic cave follows an organic lunch of lasagna, rice, and vegetable soup served at an employee's Tarzan-esque home. At certain points in the cave, the walls are no wider than an outstretched arm; a narrow rock chute is the only way out.

The mild spelunking feat is good practice for rappelling down a 65-foot rock face without a safety net. Standing on a wooden platform, you slowly back down the crude steps, then free-falls like a bird with new wings. Once on terra ferma, you zoom across a zip line that hovers above a gorge of great heights and sharp-angled depths. A waiting guide and a battered mattress cushion the crash landing.

By the time the group has zipped over—once, twice, three times for some—the sad piece of bedding has started to sag and slip down the platform. But with the rain clouds moving swiftly overhead, there's no time to adjust the padding. That will have to wait until the rain forest's next dry spell.