Best weather will probably be found on the Pacific side of the country at the end of December and the beginning of January, but this is the time that most Ticos vacation at the beaches. So, you will be competing with them for everything between Christmas Day and at least the first week of January.
You will want to have reservations during this time. Many places will require at least three days minimum, and you will be paying a premium. I would avoid Tamarindo, Manuel Antonio, and Jaco, which will all be "horribly touristy."
Consider working with a travel agent located in Costa Rica to help you locate that villa or bungalow for you. I use Ray Hagist at All Costa Rica Travel to help me with arrangements for visiting friends and relatives. Ray is a gringo, who has lived in CR for many years, and is married to a Tica doctor.
I like the area South of Dominical near Uvita or Ojochal for the holidays. Enjoy
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