Ever seen any of those promotions where you have to guess the number of jellybean in a jar or something similar? Well, you can do something similar for your Christmas party.
Decorate your Christmas tree (or the entire house) with A TON of ornaments and keep track of how many you've used. When your guests arrive they can try to guess the exact number.
Because it should be pretty hard to guess the exact number, you can give it up after a while and just announce whoever came the closest as the winner.