I absolutely loved "Secret Santa". A Christmas themed episode of "Castle" seems natural, I'm a little surprised that we've not had one before IIRC. Right from the title screen being themed to the references, and the ending, I adored this episode. I still have to catch up from what I missed...but I knew I had to watch this episode.
So I don't know if anyone has been following the news or not, but apparently in February we will finally be introduced to Castle's father in a two part episode. Also Senator Bracken will return in February as well, as well as revisiting Espo's past.