Christmas pudding travels extremely well in its bowl. Obviously Christmas cake, Yule log and mince pies would also be very portable. You could very easily take along a cheeseboard, crackers, nice selection of fresh fruit.
You could take charge of the starter, if you're having one, and transport that. Most of the classic Christmas starters would be easy to do in advance and would just need assembly at the other end, e.g. prawn cocktail, soup, smoked salmon or pate and nice bread, canapes.
You could make cranberry relish.
Delia Smith's Parmesan parsnips are ready to go in the oven and can all be prepared in advance.
Maybe prepare the sprouts and take them along ready to cook to save your parents a job?
I like to do Chantenay carrots in a foil parcel at the bottom of the oven while the turkey is finishing cooking. I top and tail them, halve any bigger ones, season, dot with butter, sprinkle on some dried tarragon, squeeze over some orange juice (or take a bit out of a bottle of good orange juice), seal the foil parcel, put it on a baking tray, cook for a minimum of 45 minutes. That could be done in advance and you could take them along to reheat at the last minute.