El Yunque (pronounced El Junque) is the only tropical rainforest that is marked as US National Forest. The park isn't so big. Enter from the North and there is only really one road, contrary to what the GPS says.
You can see some of the old towers, waterfalls (be sure to take some hikes), and lush vegetation. If you see rail guards on the road that might allow walking through, go for it. You may be delighted with the trails that lie beyond.