Book a tour with a professional guide beforehand, and ensure they have a spotting scope so you can see and photograph even the teeniest creature from far away.
Costa Rica is rife with winding rivers and Class II and III (easy to moderate) rapids readily available. Savegre is a particular favorite for beginners.
An established guide or tour company will help make you feel safe. In a life jacket and a helmet, you will be well-equipped to handle whatever the water throws at you.
These ziplines provide a bird's eye view of the Costa Rican rainforest from the security of a taut line. Bring or rent a GoPro with a helmet attachment to capture the sights.
Natural thermal baths (that don't smell sulfuric) are fed by the underground activity of the Arenal Volcano in La Fortuna and are perfect for unwinding or hanging out.
There are free springs, which are more like gentle, thermal rivers. El Choyin and El Salto are free to visit but can be challenging to find without a guide.
While Arenal is a cone volcano, the other active ones are caldera volcanoes, so they are cratered inward, often filled with water and consistent steam.