Check one of the websites of the Puerto Rico Tourism Company at www.travelandsports.com if you want to go on your own. You can also download regional planners at www.gotopuertorico.com.

Good luck.

Rent a car for sure, that is the way to go. What you pay for a tour will cover the car rental and you will have much more freedom. PR is US and your car insurance should be good. You should be able to decline all the extra insurances. You will want to see the Rio Camuy Cave park. Think about taking a snorkeling excursion from Fajardo. There are several. Eastisland excursions has several.

As you have done the rainforest and Old San Juan (but it is always worth seeing again), you could try:

Check out one of the grand hotels on a weekend evening and try your hand (feet) at some latin dancing.

Ponce on the South coast for more Puerto Rican architecture

Arecibo Observatory. The James Bond movie Golden Eye and also the Jodie Foster movie Contact were filmed there.

There is also this: Las Cabezas de San Juan Nature Reserve (Tel:787-722-5882). I have not been, but I would like to (it sounds a cool place to visit). You can have an escorted tour through the reserve on an electric trolley, but you have to book in advance.

There is plenty to do here. You might even want to consider a side trip to Vieques or Culebra (as I've heard the beaches are amazing)

Try the local food stands and just enjoy everything that the island has to offer.

Also check the online guides frommers.com, roughguide.com and www.visit-the-coqui.com/home