Rio Pacuare is the best white river rafting in Costa Rica with up to class III and IV rapids. It is also also one of the worlds top destination for white river rafting. Furthermore, you can safe yourself some money if you want to travel from La Fortuna (Arenal) down to Puerto Viejo, because the transportation from La Fortuna to Rio Pacuare and the ongoing travel to Puerto Viejo in a shuttle bus was all included in the river rafting package. I booked a tour with: http://www.exploradoresoutdoors.com/pacuare-rafting.html (external Link) but there are also many other companies which offer this. Even though some companies offer it exclusively from San Jose or Puerto Viejo and not from La Fortuna. The prices are all around the same. A new video about this great experience will be published soon.
P.S. I booked the tour with Herrera Tours Info Center in La Fortuna (2oo mts oeste de la Iglesia Catolica). They offer a little discount. Tel. 2479-8691 / 8322-1330. firstname.lastname@example.org