This is the only tropical rain forest in the US Forest Service system and one of the most scenic places on the entire island. There are several miles of hiking trails and a handful of waterfalls, including the incredible La Mina Falls. La Mina Fall is a 35-foot waterfall with swimming allowed in the pool at the bottom.
If you go:
La Mina Fall is a 35-foot waterfall with swimming allowed in the pool at the bottom. The falls are accessed by one of two hiking trails, La Mina Trail or Big Tree trail. Each trail is paved, somewhat steep, and about 3/4 of a mile in length. One trail starts above the falls, the other starts below, and you can make a full loop by taking a one-mile walk along the main road through the park. La Coca Falls, which can be seen from the main highway, is the other major waterfall in the park.
Links:

Puerto Rico Route 988
Sabana, Puerto Rico 00721