I certainly do not have  the experience or knowledge of Costa Rica that Bruce does, but my six winters down here (I'm in Costa Rica now) have given me a little insight on the richness of this country and the impossibility of covering all the "you must visit" parks and hotspots in a short period of time - how about a lifetime!  I stay in the pacific Northwest and make excursions from here.  If you are targeting certain species of birds I do recommend the book A BIRD FINDING GUIDE TO COSTA RICA by Barrett Lawson.  It lists birds by species and provides a chart as to where one might locate a particular bird.  If that is not your goal, I certainly concur with Bruce's suggestion that you pick up a 10 daybirding tour. I cannot recommend one over the other.  There are others on this list who have done that and always seemed pleased with their experience.  Perhaps they will comment on this thread.  If you do not like the idea of a tour, then I suggest you zero in on a particular area like the Osa Peninsula or the Caribbean/ La Selva/Sarapiqui area and focus on a smaller area rather than the entire country. Good luck!Cheers,  Martha Pfeiffer in Ocotal