The paper crane is a very traditional shape and perfect for filling in your tree's bare spots. The crane also symbolizes world peace along with the Japanese legend that states any person who folds 1,000 gets a wish. Very holiday-ish I think.
If you want to spend money on origami paper, it is available at craft stores. I like to use regular paper or even magazine pages for a glossy, eclectic look. The only thing you NEED is a square. Animated directions are found here.
Definitely experiment with the sizes of paper. While the larger cranes look great sticking off the edges of the tree, the smaller ones are too cute nestled in the branches. I like to follow the "odd" rule. Everything looks better in odd numbers.
I also thought of making a wreath out of brightly colored cranes placed snuggly together. Use a foam form and attach your little winged friends with a stick pin. Maybe next year :) Happy Folding!