I made the hat out of A4 green paper, cut in half and the ends taped together with plain office tape and all taped into a nice shape. It would fit in the head hole to cover it up, so when someone arrived I got people to stand, covering the arm holes and tell the person that it was their Kris Kringle present, then I would jump up and give them a shock. Hilarious.
My costume was 1m x 0.5m, and covered in about half a roll of wrapping paper. It only cost a few dollars and was really fun to make. The only downside is that you have no arm control!