Let me just say, we're not a jigsaw family (me, DH, teenage sons.) We went for a weekend away and I accidentally bought a jigsaw game, thinking it was just a murder mystery game. We spent hours on the puzzle! Altogether and separately coming back to have a go, it was a lovely communal thing! We're going to see if there is a Christmas version from the same brand, (Goliath brand. Scene of the crime, the stolen necklace mystery, was what we had )
Exploding Kittens is a card game that can be over within 15mins for quick rounds. It's really fun using the cards as a fun quip to force the other person's hand to be the one to get the Exploding Kitten card.
Re: decor/Christmas feeling: plug in air freshener or maybe a small febreeze with a Christmas scent if plug ins are too strong. Spiced Apple or even cooking things like ready made cinnamon rolls etc, to naturally scent the room.
I'd take some lights, drape them or tape them somewhere, are you taking a Christmas tree? Argos has a cheap, small 3ft Tree also, is there anything that will be local to you once you're there, for foraging? Could you all collect a few branches/acorns etc and using florists wire, all make a small wreath for the door or garland for a particular area? I'm thinking something that doesn't need packed or taken back home with you, easy to dispose of after removing the wire... Christmas teatowels/mugs?
Christmas music playing while someone prepares breakfast is a nice thing in the morning. Low enough volume not to wake anyone but festive for anyone coming for a coffee. Doesn't need to be the more lively Christmas songs, I'm thinking of abit of Frank Sinatra, calm songs.
Perhaps make a surprise festive snack box/board. Stuffing sausage rolls with turkey, mayo, cranberry and stuffing sandwiches, Clementines etc. Will attach pic of fancy boards that look good.
Re: gift giving... If you are gifting vouchers, my first thought is to put a voucher in a bauble, the type that open and close. But depends if you a) have them and b) somewhere to put it. Cheap stockings? Anywhere you can hang them? Recreate the feeling of excitement by having a stocking full of mini gifts? Socks, Chocolate orange, mini alcohol bottle, the voucher inside a mini Christmas card, all individually wrapped to draw the process out longer?
Sorry, perhaps got on a bit of a tangent there!