Guanacaste is the dry region of Costa Rica, as you will soon find out. There are not many rivers up there, and I have not heard of any whitewater there. However, there is some good stuff near Quepos and farther south (Rio Naranja III-V, Rio Savegre III-IV, and the Rio General IV). Otherwise, there is all the stuff on the Caribbean side, near Tubbialba (Rio Reventzon, Rio Pacuare, Rio Orosi etc...)
The Genral is a great multi-day run. Naranja is nice IV- short run. The Savegre is supposed to be nice, with a multi-day option. The other stuff near Turrialba is a no brainer, its all awesome.