mummytron how about wind up things to have races? Or toy cars to have races? One year I did the little pull back and go camper vans that Sainsburys sell for £2.50 (but mine had been marked down to 25p!) and had sharpies out so people could customise - and then race, which was fun. I've bought the reindeer toss game from previous threads for this year - fingers crossed it comes.
Selfie props for silly pictures? Poundland have notice boards likes speech bubbles to write comments into your selfies.
Origami set?
Craft set? The works has all sorts of sets and kits and games and puzzles.
Home bargains has tissues and napkins printed like £50 notes (other denominations are available)
Finger lights?
Small musical instrument like harmonica, castanets etc
Job lot of rubber chickens and do a diy Chicken Charades
Asda £5 Xmas fleece to snuggle in after lunch
Tea towel for the unlucky ones doing the washing up 
Silly hat (search cow hat on eBay!)