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.
Backyard Chalkboard - paint and mount an outdoor chalkboard on a boundary fence for a great activity for children. Less mess + the rain washes it away! Add some simple wooden shapes in a bucket so kids can trace around them & create masterpieces.