I am sure you will be visiting the Monte Verde Rainforests in Guanacaste. The city of Guanacaste is pretty cool, spend some time there, good food.
We went to a place called Selvatura, basically it is just a company that has put up huge suspension bridges and run trails throughout the forest. There is also a canopy tour there where you ride around on zip lines hundreds of feet above the forest. We saw one of the most rare birds in Costa Rica and in the world there.
We also spent time at Arenal (The famous costa rican volcano and one of the most active in the world) and Manuel Antonio national park (in Quepos/Manuel Antonio) where we stayed at a place called Parador (http://centralamerica.com/cr/hotel/parador.htm). This was the nicest place I ever stayed, and this area was my favorite place.
If you have a chance to make it to Arenal of Manuel Antonio (get a guide here) I would recommend it although they are both several hours from Guanacaste.