For your last day I will advises to return quietly to San Jose in order to take your return flight. However, if you are still enjoying the day and your flight happens to be late in the day, take the opportunity to visit the capital which is San Jose or for go see the Poas Volcano.
This volcano is located towards Alajuela, which is also where the international airport is located.
It is a volcano still active today which is worth the detour when the view at the top is clear.