Costa Rica is justifiably famous for its rafting. You have the choice of doing day trips to a number of Class III (intermediate) rivers. From San Jose, you can do the Sarapiqui (Class II-III), Reventazon (Class III), or Pacuare (Class III-IV) rivers.
Also popular is to do a rafting trip over a couple of days, with a stay either in a nearby hotel or at an overnight jungle river lodge in the middle of two days of rafting. The latter option is done on the Pacuare River, with several class IV sections. It is a fantastic experience and one of the highlights of rafting in Costa Rica.
From Arenal Volcano, you can do a full day rafting trip down to the Sarapiqui (Class II or III rapids). From Manuel Antonio, you can do a full day rafting trip to either the Savegre (Class III, all year) or the Naranjo (Class II, running only from April to December).
Canopy tours are an exciting way to experience the rainforest up close, at the level of the branches of the tallest trees. The canopy tours vary. The most adventurous are the zipline canopy tours, where you are put in a harness and sent along a series of suspended cables between different parts of the forest. There are popular zipline canopy tours in Monteverde and in Arenal. In Arenal, you also have the option of doing the Arenal Hanging Bridges, which is a series of suspension style bridges hanging between different sections of the rainforest, that allow you to experience the biodiverse canopy level of the rainforest up close, but while staying on your own two feet. The Skytrek Canopy Tour in Monteverde is a beautiful canopy tour where you walk along suspended bridges at the level of the lush cloud forest.
Costa Rica is one of the great places in the world for sport fishing. Offshore fishing is generally good all year round (except for October and November) for Sailfish, Blue, Black and Striped Marlin, Yellowfin Tuna, Dorado, Wahoo, Roosterfish, and Snapper. For bill fishing, it is best from December to April off the Manuel Antonio/Quepos area, and from April through September off Guanacaste (Tamarindo). There are strong winds off Tamarindo and the northern coast from December to April, and offshore fishing is not that common during this period. However, fishing is still possible closer to shore and a bit further south along the shore. Near the Osa Peninsula, in the far south, there is also excellent billfishing, however more emphasis is placed on the inshore species in that part of the country.
Scuba diving in Costa Rica offers a great variety of marine life, but the visibility is not what you will find in other places, for example in Belize. However, you can enjoy views of a tremendous number of species, including sharks, rays, and huge schools of many other kinds of fish. The best diving off the coast of Costa Rica takes place off Guanacaste, in the north east of the country. The Catalina Islands are considered one of the best places you can go, and they are accessible as a day trip from Tamarindo. In the Osa Peninsula, diving is popular off the Isla del Cano, and the visibility is better than at many other places in Costa Rica. Snorkeling is also popular along the coasts of Tamarindo, in Manuel Antonio, and off Isla del Cano.