Looking at the Punta Islita Guanacaste. I've got Marriott points/status, so it looks like a decent option for us. Not cheap, but prices are sky high for the week after Christmas, so the redemption value is ok.
@platamama stayed there (at least once) and I think she was trying to plan another trip to go back. I haven't been there, but we stayed elsewhere in Costa Rica years ago and absolutely loved it. Beautiful country, really nice people. Pretty easy to get by in English (I speak bad Spanish, DH speaks NO spanish, and he was able to happily spend an afternoon looking around the small town near our resort). Gaah, I've gone from "huh, I wonder if we can do a trip in December" to "Oh my god, this trip had better work out" in a matter of hours