Is rainy season over by Christmas time? I specifically avoided the rainy months and went at the end of March. But you might be ok in late December. Something to double check though.
We flew into San Jose and rented a car (used Southwest points). Driving was a little tricky due to poor signage, but we had a GPS and managed ok. They also have shared shuttles as a way to travel if you don't want to drive. But for 4 of us, a rental car was cheaper + we like the freedom.
From San Jose, we drove to Manuel Antonio National park. Stayed here 3 nights (2 full days) and booked with UR:
https://www.fallsresortcr.com
In Manuel Antonio, we did a park tour also booked with UR. It was inexpensive and very worth it (got to see wildlife we would never have spotted on our own). The beaches in Manuel Antonio National park are beautiful + its so much fun to watch the crazy monkeys!
On day 2,
we drove up the coast and did horseback riding on the beach - also booked with UR. We were the only 4 on the tour and it was TERRIFIC!
http://www.theridingadventure.com
Its a beautiful country; the people are friendly. And its fairly easy to travel on your own. I highly recommend!