It depends on where you're going, but in general May-November is considered the rainy or green season. We still travel then because you can usually get good discounted rates on hotels. The most popular itinerary for first timers is Arenal and Manuel Antonio. Other popular areas are Monteverde, Tortuguero, the Guanacaste beaches, the Sarapiqui, and the Osa Peninsula. Most people don't spend a lot of time in San Jose, maybe just a first or last night due to flight times. The real beauty of CR is out in nature and exploring the wildlife areas, beaches, volcanoes, etc. You can find accommodations in just about any budget and price range. Do a search on some of those areas and see if they appeal to you. With a week, you probably wouldn't want to visit more than two locations.

My advice to most people in your position is to buy/read a good guidebook on Costa Rica (Fodors obviously, but Moon, Frommers others are also available).

Peruse it and see what jumps out at you as a "wow I'd love to see that" destination.

Manuel Antonio and Arenal are the most common for first time visitors, but there are a lot of other choices that could suit you better. Without knowing what you're into--wildlife vs beaches vs rafting will produce radically different destinations--hard to say.

Time of year--that's another moving part--some parts of the country are too dry during the dry season (Guanacaste) and some are entirely too wet during the wet season (Osa peninsula).

It might be helpful to plan a vacation time by answering the question "when do I want to leave home for a week" and then planning around that time frame, your interests, budget, etc.

Hey Mick, I will meet you in Mexico!