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.
Easy kiddie kitchen sinks: find a table, cut two holes, insert plastic tubs - can't find this on the link but the picture pretty much tells you what to do! Great idea for outdoor play space In the summer