This is my favorite hike by far. Cueva Ventana ("Window Cave") is a beautiful, scenic hole in the side of a mountain that opens up to a lush, green valley down below. You'll trample through mud, hunker down under low-hanging stalactites, dodge around stalagmites, and never look up (there are bats, of course) while wandering through the cave.
All this winding around through two caves leads you to one of the most spectacular views in all of Puerto Rico (but I may be biased).
The hike to this cave is half the fun. You climb down into a small opening under enormous, ancient tree roots, and you get the feeling of a tomb raider. But, if you're claustrophobic or don't feel up to lowering yourself down a cave wall, there's an additional path that bypasses the first cave and leads you straight to the easier hike to Cueva Ventana. There's something for everyone! I'm pretty sure I saw one Puerto Rican woman doing the whole thing in heels.
This is a must-visit site for anyone touring the northwest central part of Puerto Rico. Another tidbit is that it's practically free! You can find the directions on PuertoRicoDayTrips.com, park at the adjacent Texaco gas station.
UPDATE (2014): There is now an easier way to access this awesome view! There are now boardwalks for ease of access, and you must pay $11 to tour the site. You get a flashlight and hardhat for safety.