QBitz is a nice game for all the family - you can buy 1 person boards, or a full set containing 4 boards but still play just individually. So it would be handy for on a plane to entertain themselves (and practice!), but then all play together in a cabin over holidays. Maybe put a few cards each and their own board into a Ziploc sandwich bag each for the flight?
Travel Monopoly works well!
We also have magnetic ludo, snakes and ladders, and chess that DD likes for travelling, and small "connect 4" games (1 is from Tiger, another from a £shop type place).
Seasonal edition of a favourite magazine? As opposed to an actual seasonal magazine?
Some really nice detailed pictures to colour in, and nice colours/pencils/markers for the trip - activity village do nice ones, and lots of other seasonal ideas there too, many of which I've "borrowed" for DD when we've travelled over Christmas.
Craft on planes could be tricky - no solvents (pritt stick glue might be ok but include in your "liquids" bag). No sharp items - scissors, needles, knives etc all ruled out.
For when you get there, a stapler or roll of tape, and a bag of strips of different coloured paper can be used to make paper chains? But they would probably not travel well once actually stuck together (I know we've often done that as something to keep DD busy on arrival while we unpack and settle in).
A small festive stuffed toy? A little reindeer or santa or similar? Or festive outfit for a favourite doll or teddy that is coming as well?
LEGO!! They do lots of Christmassy stuff, and a few things are stocking filler sizes or small kits that could work well on planes.