It’s hard to say what I like the most about Costa Rica. There are so many fun things one can do: watching sea turtles lay their eggs on the beach, humpback whales dance in the sea or to swim with dolphins. If you don’t have the time or come at the right period (especially if you want to see the turtles), you can always visit one of the many Costa Rican volcanos. Some of them are very close to the capital and easy to access.
Volcano Irazú is one of them. It’s located above Cartago city (an hour drive from San Jose) and easy to access. I recommend avoiding Sunday as that’s the day for Costa Ricans to visit national parks.