If you have time for only one side trip, this is the one to take. Just 35 miles east of San Juan is El Yunque National Forest, the only tropical rainforest in the U.S. National Park System. Named by the Spanish for its anvil-shaped peak, El Yunque is a photographer's and hiker's paradise, with waterfalls, wild orchids, giant ferns, towering tabonuco trees, and sierra palms.