El Yunque is a treat. The only rainforest in the U.S. National Forest Service, it is perenially a verdant, peaceful oasis, and a wonderful place to go hiking.
But that doesn't mean it's necessarily a day in the park. The forest has a network of trails, from the relatively easy La Mina (which takes you to a waterfall) to the more challenging El Yunque Trail. Whichever one you choose, bring good shoes and a picnic lunch, and enjoy a day under the forest leaves.