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.
Gabion walls don't need any kind of drains because they are porous throughout. Gabions can be customized to make them more useful in public spaces. Gabions can be filled with any heavy material--diverse stones, metal artifacts, obtanium, etc.