I live in Turrialba. I'm sure you'll be coming here as the Paquare, the country's most famous river, is about 30 minutes away. Regardless, I can help you with a few things if you'd like.
I don't know how much you've planned, but I'm sure there are three things you want to do: kayak the Paquare - and probably the Reventazon, see the volcano at Arinal and visit the beaches, especially the Caribean.
My girlfriend's parents own a bed and breakfast/restraunt/gallary in Arinal so that might be an option for you when you go to visit the volcano. (It's active, so that's why it's such a big deal.) I can also get you hooked up with transportation from Turrialba to the river if you'd like suggestions.
As far as the beaches go, I like small towns so I spend my time at the beach in Manzanillo. It's VERY caribean. The people are big and black and love to feed you! Unlike the places you'll visit while boating, the blacks speak English, so it makes everything eaysier. The Pacific side is very Gringo at this point but very much worth your time and again, everyone speaks English.
Let me know if there is anything I can do to help you out.