Whether you are visiting or live in Costa Rica, most people spend at least some time in the capital city of San Jose in the Central Valley area.
Many visit the San Jose downtown sites and are then looking for nearby places to visit for half or full-day trips. Most popularly mentioned ideas based on their relatively close geographic proximity to San Jose can still be several hours in travel time due to Costa Rica's terrain, roads and traffic. Examples: Poás Volcano is 1.5 hours away, Arenal Volcano is 4 hours away, La Paz Waterfall is 1.5 hours, etc., etc.
A closer and no less interesting destination is San Jose's neighboring city ... Cartago.