Make a wind chime out of old keys and acrylic paint. - This site has lots of cheap outdoor/backyard ideas. I can't use most because I live in an apartment - but I love the old key to windchime and spray painted folding chairs.
Elf house on a tree! That would be cute in the garden, especially in the tree over grown with ivy. How about some battery tea lights in the windows during outdoor parties. I hear the fairies/elves whispering "home school project" in my ear.