Cambalache Forest Reserve protects a patch of north-coast forest east of Arecibo, with trails used for hiking, mountain biking, and birdwatching. It is not a manicured attraction, but it gives you an easy way to trade the coast road for shade and birdsong.
The reserve works best as a short nature stop if you have a car and are already exploring Arecibo's caves, lighthouse, or karst coast. Bring water and expect simple facilities.