Nothing gets you in the Christmas spirit quite like a sing-a-long. This particular fundraiser would work well with a school choir or glee club and can bring a smile to the faces of the participants and the audience members while you raise money for a good cause.
Divide up your group into teams of 4 or 5, keeping one adult in each team for safety reasons. Put together a flyer that explains the purposes and aims of your group and what the money's going towards. Dress up in Santa or elf hats if you can find these props and make sure you have plenty of song books to go round with popular Christmas and Holiday carols in them.
As you tour round your local neighborhood, knock on doors and start singing. Be respectful of any residents who aren't interested, and take requests from those who want you to stay. Give out the flyers you've prepared and emphasise that donations are optional.
Make your main aim spreading Holiday cheer around your area and treat any donations you may get as an added bonus. Obviously the closer you run this fundraiser to the Christmas holiday period, the more successful it's likely to be.
You can find more Christmas themed fundraising ideas here.