This is a must in Puerto Rico! The rainforest is located right by Fajardo. There is a visitors center with a great view and information and many hiking trails. We climbed to the waterfalls, where we had fun splashing around in the water. Its not a strenuous hike, but does take some effort if you are not used to hiking. We had no trouble at all!
If in Fajardo, take a snorkeling trip on the "East Wind" catamaran which comes out of the Villa Marina in Fajardo. For $69.00 they take you out snorkeling to the "Spanish Virgin Islands" which is off the coast of Fajardo. The trip leaves the dock at 10:00am and returns at 3:30. They provide all gear, floats (for non swimmers), a great buffet lunch and unlimited "Pina Coladas' and "Rum Punch". The crew is knowledgeable and you will have a great time
If you are up for an adventure, then take the ferry to Vieques or Culebra. Make sure you check into the weather first. We went out in July 2008 when there was a tropical storm brewing just over 1000 miles out, but did we feel it. As soon as we got to Vieques, we made plans to take the same ferry back to Fajardo. It was just me wanting to feel safe while the sun was still out. I wish I would of stayed in Vieques, and discovered.