Just moved from Costa Rica. (I lived there for a year.) It is not "AS MUCH" of a third world country as some others in Central America and you can get most anything you need. Buses are cheap but the roads are lousy and it is quite mountainous so if you are prone to motion sickness bring your dramamine or buy Gravol in country.
West Coast is nicest. Try Tamarindo, do go to los lagos at Arenal and if you can swing it go deep sea fishing in Quepos. Manuel Antonio is the nicest national park on the beach but stick to the third beach there since the undertow is HUGE at the other two. LIMON is NOT a great place and many of the Atlantic areas such as cahuita have a heavy drug presence. Jaco, on the West coast, has great surf but is not a great place.
While you are in San Jose DO go eat at "La Casa de Dona Lela" which is just outside of town on the road to Limon. They have the best typical food in town . . . get one of everything!

Or at least "Trocitos de Lomo al Sarten"!
IMHO, DO NOT drink the tap water! CR has recently become famous for allowing their sewage lines to mix with water lines and last year even the locals stayed sick pretty often. I drank it for the first five months I was there and then got Giardia, Ecoli, Bacterial Infections, Amoebas, and Parasites traced to the water within the next two months. They have begun changing the tanks an water lines and most of the American groups there now do not recommend it.
Nicest american stle hotel, for me, is the Marriot courtyard in Escazu right outside of town. Downtown hotels tend to be loud, surrounded by significant crime and prostitutes galore.
CR is an eco-sensitive country so unless you are down with a guide on the osa peninsula, you won't need a machete or large knife. Take a multi-tool, and a swiss army knife. The local laws only allow for a 9cm blade on a folder but it can be an auto. Do take raingear and a good umbrella as well as plenty of bug spray. Right now is the heart of rainy season so expect rain to begin at 1p.m. daily! Mosquitos are only REALLY bad at the beginning and end of rainy season and Dengue fever is not common there.
There are TONS of english speakers so language will not be as big of a barrier as you may expect. Have fun!
EDITED TO QUALIFY SOME STATEMENTS and correct my poor spelling!