El Yunque National Forest (Puerto Rico)
The biggest piece of advise I can give, if you're interested in exploring El Yunque National Forest is DO NOT PAY FOR A TOUR! Rent a car, or Uber there and explore it yourself, at your own pace.
The drive there is easy (from San Juan) and there are many shops and restaurants to fuel up before entering the park. There are also a number of snack bars throughout the park at many of the trail-heads and parking lots.
When you first arrive at the park, go visit the visitors center. Here you will find everything you need to know about El Yunque National Forest, which is the only rain forest in the US National Park system. The visitor center, as well as the park is free and you will find maps here, helpful park rangers to guide you to all the parks destinations, a gift shop, a snack bar, a short film to watch and a small museum type walk through that will explain the history of the park as well as the animals and birds you will find here.
El Yunque National Forest is best known for amazing waterfalls that you can hike to and swim under. The hikes to the two main waterfalls, La Mina and El Coco are under an hour and are paved fairly well. For more experienced hikers, you will want to hike to El Yunque Peak where you'll see amazing views of the whole island (on a clear day, it does rain here everyday).
There are also two lookout towers you can climb for amazing views as wells as observation points along the only road that goes up and down the mountain.
Bring some old shoes that you can hike in and then trash when you're done as there are spots that are muddy and you will get wet. If you plan on getting in the water, I would not recommend going barefoot as it is very rocky.