Jaime C stop by at Lake Arenal if you have time!
John Brine, forget about city and town, off to the lake and the coast! If you just plan to visit the beaches fly into Liberia to dodge San Jose, it's terrible there. My recommendation; no resorts and forget about the popular tourist places like Jaco, Tamarindo, Coco, Quepos/Manuel Antonio. Rent a car and drive around without a real plan. Stay longer if you like it somewhere, move on when you're done. If you have the time, visit Peninsula de Osa (Corcovado National Park, Drake Bay) in the south (maybe fly there cause it's a long drive and flights are cheap). Again, time permitting; visit the Caribbean coast (Puerto Viejo/Cahuita) and spend 2 days or so in Tortuguero (north caribbean). Arenal lake and volcano should not be missed. Monteverde in case you couldn't make it to Osa. Then on to Peninsula de Nicoya. Surf, relax, and drive from Nosara along the coast down to Montezuma (or vice versa). I'll pass you some places where to stay along the road. This drive is one of the coolest things to do in CR, I think.
Any questions just shoot!