A big jigsaw, something that can keep you going a few days.
I'd walk through the city centre anyway, with a camera, looking for interesting things to snap (cat on a windowsill surrounded by little lights, etc, anything quirky). You're sure to see people out and about who don't celebrate Christmas, or walking dogs, to say hello to.
You could watch Die Hard and play a drinking game - one shot for each F-word , one shot if you say 'yipeekayay motherfucker' at exactly the same time as Bruce, two shots for the first person to notice his vest change colour halfway through a scene 
Pre-xmas look up some cheesey Xmas jokes and tell each other different ones over the days that feel the longest. You could fold them up and pull a joke out of a box at random whenever you feel like it.
Cook your favourite meal - we don't have traditional UK Xmas lunch (abroad, can't find half the ingredients), so have something rather faffy but tasty instead. I make crackers out of bogrolls. I put a joke and a small gift in each, often a cute pen (I have DH and ds), a small cute toy from Flying Tiger or whatever. This year DS has got a small telescopic backscratcher, something he's been mithering for but for a euro it's ok to go in a cracker. I need to use a kitchen roll tube as it's a bit longer than a loo roll.
You could make an online album or Hoffman album or scrapbook (whichever floats your boat) of lovely positive things that have happened this year. Whether it's a pic of pub lunch with mates, you both at the cinema, random things that have made you laugh during the year.
I'd follow zooms calls with a cup of tea and look at the birds in your garden (if you have a garden...and visiting birds! Maybe make fatballs for them Xmas Eve and hang out). Or put a film on - doesn't have to be festive.
Big hugs, Xmas can be a fucker x