Renting a car is cheap and easy. Most rental cars are very cheap compared to a taxi and it is very easy to drive in PR when it is not rush hour. Stay away from the weekday rush hour traffic.  This was the best way to visit the rain forest.  They have a lot of theft from cars in the rain forest so do not leave anything in plain view. Keep cameras and stuff with you or in the trunk. Drive all the way to the top and then hike to the highest point first. This will take a lot out of you but it is the best. Taxi's and tour buses can take all day but you can do the rain forest in a lot less time if you rent a car.