A damn good Tankmas ADVENTure - no clue when the "Holy Crap A Yeti" arcade game's supposed to be unlocked, but its getting pretty late in December as I'm writing this. Not sure if its broken or not.
Update: Played a few games of "Holy Crap A Yeti", and god damn is it hard - even when you aren't losing your streak to bugs, you're still juggling three different things at once.