Almost every Costa Rican vacation starts and ends in the Central Valley, otherwise known as the Meseta Central, where the capital city of San Jose and the more affluent neighborhoods can be found. It is the country's premier cosmopolitan center, with conveniences like supermarkets, malls, hospitals, restaurants, and such just nearby. It is also home to some cultural institutions like museums, parks, gardens, and major libraries.
Included in the Central Valley Tourism region are San Jose and the areas surrounding it. This includes the Juan Santamaria International, the entire San Jose province, and parts of the Alajuela, Cartago, and Heredia provinces. Much of the country's population is concentrated in this area, with other major populated cities located here.
Do not think that it's all about cosmopolitan stuff that you can find here, which might even have better versions from where you come from. There are still a lot of natural destinations that you can check out, such as the Irazu Volcano, Poas Volcano, Braulio Carillo National Park, and many similar others.
By visiting each of these smaller communities and natural attractions, you will get yourself more absorbed to Costa Rican culture aside from that one featured in the capital city. Spanish and pre-Columbian cultural influences are still very much prevalent here. Costa Rica is all about cultural diversity, and it would do you well if you decide to visit each of the outlying regions.
Furthermore, the Central Valley's, well, central location makes it a very favorable spot to set up your base in the country, while allowing you to visit the farther provinces up north and up south. Wherever you intend to go, the Central Valley is the best point from where you can start. From there, you can already take a bus trip, which is sometimes unbelievably cheap, and proceed to your destination.