Manuel Antonio and Arenal are the most common for first time visitors, but there are a lot of other

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 tv travel shop for first timers is Arenal and Manuel Antonio. Other popular areas are Monteverde, tv travel shop 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.
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.