I'm from Costa Rica, so I can recommend you a pair of good places!!!
If you like beaches, Guanacaste it's the place to go!! Tamarindo it's one of the most beautiful beaches in Guanacaste, but you can visit also Conchal (entirely covered by sea shells); it's really close to Tamarindo, and Playa Grande (between Conchal and Tamarindo) it's beautiful too!! These are located at Las Baulas National Park, where baula turtles go to put their eggs every season.
If you like a special place, there a place called Rio Celeste (Blue River) where you can find a incredible blue river, and many goods family hotels; here http://www.riocelestehideaway.com/
you can find some pics, and there's a lot of tourist guides who can give a nice tour.
If you like volcanoes, the best one to go its the Poas volcano : it's the biggest crater of the world, and you can see the crater very close!! This is how it looks https://www.google.com/search?q=poas...iw=768&bih=928
Another beautiful place it's the Arenal Volcano, where you can see the Arenal in eruption... It's a beautiful show, and unforgettable http://www.arenal.net/arenal-volcano.htm
If you like to go and see some music, the best place to go its Jazz Cafe!! The have shows almost every day, and you can see really good musicians playin there!. There are two,locations: the one in Escazu has the best location, its the nicest place, but I enjoyed much more the San Pedro location!! If you can catch these two bands "Swing en Cuatro" http://youtu.be/LWpDdoSceNQ http://youtu.be/nqoeAcVAa1E
or "Escats" http://youtu.be/dLPPt89zM3Y
on your visit, will be great!! These are the best bands around, and have incredible musicians!! This is my former band, playing at Jazz Cafe Escazu, some time ago... http://youtu.be/8DzAj53eTRg
Tell me what you want to do, and maybe I can recommend you some place to visit or stay!! Hope this helps!!