What would nurture them on Christmas Eve, as they settle down before the mad rush of Christmas Day?
Christmas/winter beer, or a miniature of a favourite tipple?
Puzzlebook, Christmas crossword, Christmas edition of a favourite magazine (Spectator, Phoenix, a weekly sporting or monthly history or gardening journal??)
A nice pen/pencil to do any puzzles?
Nice bubble bath or shower gel or bar soap - a proper "manly" one but maybe festive or bit more luxury type for DF - travel sized ones are great for CEHs. You already have something for DM I see.
Definitely something cosy like fluffy socks, slipper socks, slippers, etc for both to relax.
Something small but special to eat - a favourite treat from years gone by, something they like just at Christmas, or something new to try.
Maybe a nice movie to enjoy together, or a book each and a CD of music they would enjoy listening to.
Not everything - just a few ideas to choose from.
And maybe put a handwritten note in as well, to explain it and, more importantly, that you have noticed them doing so much for others this year and that you wanted to do something special for them as well.....sort of message