It might be "old school", but what kid doesn't like drawing on a blackboard. And when they are finished, give them a bucket of water and a paintbrush to wash it off again ready for next time. We've got lots of other ideas for kids on our main site. http://theownerbuildernetwork.co/ideas-for-kids/
I like the idea of an outdoor chalkboard so kids can color without being out front of the driveway or blocking the patio when people are over. If I put it under the deck, it would even be protected from washing clean right away.