O.K., it's a little out of the way (if you take the coast road from SJ, it's nearly to Croabas/Fajardo). But this is one place you can't afford to miss. If you're staying in San Juan, you'll probably have to rent a car. Admission is $3.00. There are tours, but they will greatly restrict your opportunity to explore. (This is no place to be in a rush.)
We took the Mt. Britton trail (moderately strenuous, but only a couple of miles long). It had lots of ancient tree ferns and orchids. You'll probably want to stop at Las Cocas falls (right on rt. 191), and if you take a tour you should go to Las Minas Falls, so you can swim. Wear a swimsuit under your clothes if you go to las Minas. Trails are well marked and well maintained. Good rainforest. Good visitors' center and souvenir shop. See the Jimmy Smits narrated film. A great place to walk with your kids. We saw a rare Puerto Rican parrot in a tree. Loved the whole experience.