Sidewalk chalk is one of those classic toys for kids. The mess is usually easy to clean up.
There are some really fun types of sidewalk chalk now, such as some that comes with glasses to make projects look 3-D. It’s a really nice effect, although not utterly necessary.
You should be a little careful, of course. My kids have been known to drawn on exterior house walls or on the fence, and it doesn’t come off those surfaces so easily. If the kids are out front, you should also consider how neighbors feel about it. Some people can be touchy about chalk drawings staying out longer than the kids who drew them. There are times when it’s more polite to hose down the drawings promptly. I think it’s silly, as chalk goes away pretty fast on its own, but others disagree.
For those who are really into it, there’s a recipe for ice chalk that also looks like a lot of fun for the kids.