Arenal Rafting works the rivers around La Fortuna at every grade from family floats to class IV, and sells combinations with ATV riding or canyoning for people trying to fit a lot into a short stay. Trips start around sixty-five dollars including lunch, gear and hotel pick-up.
Their site carries a WhatsApp button pointing at a landline that is not the number they publish for contact, so use the mobile above and confirm the channel.