El Yunque is the only tropical rainforest in the U.S. National Forest System. Although you can explore yourself without a guide, it's best to book one for your first visit so you can learn about the importance of the rainforrest and enjoy the most popular spots. If you do end up going without a guide, I recommend stopping at El Portal Visitors Center which acts as an information center and also has a cafe.
Kia's favorite beach also happens to be the one I grew up going to the most as a kid. Grab a boogie board as it tends to be wavy. Just be sure avoid La Punta (the point) which is towards the end of the beach and full of undercurrents.