The least expensive Christmas tree decoration I know is to use lawn harvested pinecones. Pick up any size or kind from the ground (ask permission but most are happy to get rid of them!), take them inside and let them warm up and open up. I put mine in a plastic bag so any critters are contained.
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What I really like to do is, collect pine cones. Then put them in a low oven, about 225 degrees F. for 15 minutes or so. Then I dip them in melted candle wax that was left over from candles I burned. I then sprinkle them with glitter...