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.
DIY Kid’s Play Platform and Jumping Stumps! Have monkey bars coming off the side and a sandbox underneath. Have rings and trapeze off back and climbing rope hanging on last open side. Have rock wall instead of ladder.