Growing strawberries (Or any plants!) from a gutter hung on the fence drilled holes every few inches in the bottom so they would drain well, and added Miracle Grow, and planted 100 strawberry plants in 10 foot long metal gutters
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.