I wish the search feature on this site worked a little better as much has been said in the past about Costa Rica.
I would not focus on classification as significantly... I spent a few days paddling in Costa Rica and enjoyed the class 2-3 as much as the 3-4+ stuff. Generally because the experience of paddling there is so unique. For example the Pejibaye was one of the coolest experiences i have had yet. The whole river was covered by canopy with beautiful blue water, So amazing. The Pacuare is really big water so that adds to the flavor but very dirty (lots of mud). To compare the Pacuare felt alot like the Gualey. the scenery there was really cool as well. I did not make it onto the Reventazon, it was on my list but I got hurt. I was helping a friend portage and twisted my knee.
Rent a boat. Dont take one, it helps the boating community and hire a guide or go with a raft trip and pay for it, again it helps the boating community there. This is important as they have been trying to dam the pacuare for years.
Turrialba is kind of the Mecca of Cost Rica boating. Most of the rafting companies are there and most of the boat rentals are availabel there as well. Its a cool little town and worth the visit, based mostly on coffee but tourism is having its impact. We stayed at theHotel Interamericano
(click the link) its a hostel, not luxurious by any standards but still very clean $12 a night. So your friends may dig a raft trip and may even like the feel of turrialba.
On a side note I would also encourage a visit totabacon
was another one ot my highlights of a trip to Costa Rica. By far the most beautiful place I have ever been. It is a hotspring runing through a river bed. Several water falls that you can sit under with the water on your shoulders and such. Then afterward we went to Volcan Arenal and wathced Volkswagon sized Lava Boulders shoot out of the Volcano and down the mountain. Freaking awesome. We did not stay at the resort as we did not want to shell out that cash.
So there I am getting sick of talking about this, makes me want to go.
If you have any other questions feel free.