This boat ride leaves from Selva Verde Lodge in Puerto Viejo de Sarapiqui, and there is a neat suspension bridge you can walk across and a lunch offered. Then you get into little passenger boats and head through the jungle down the river (which is right near the Nicaraguan border), with guides who explain all the cool wildlife you see along the way. It is relaxing and exciting to wonder what types of wildlife may be around the corner. I remember seeing toucans and sloths, and perhaps a croc (scary!)
This was all part of the "Costa Rica Highlights Tour," which included breakfast at a mountain lodge, a Poas Volcano crater visit, a walk through a cloud forest, a stop at La Paz waterfalls and hummingbird gallery at Braulio Carrillo National Park, lunch at a rainforest lodge, and then this 1.5-hour jungle river boat cruise, where we saw monkeys, iguanas, crocs, and various birds and sloths. It was run by Ecoscape Natural Tours, 506/240-5106. I highly recommend it if you are short on time and want to see everything you can!