All you need are:
paintbrushes- any kind work
Just have the kids dip the paint brush in water and then they can paint the sidewalk or driveway with whatever pictures they want. If it's a hot day (as if there are any other kinds in the summer), fill up a kiddie pool and let them them play in the water and then they can use that water to paint pictures. They'll never run out of space because the picture dries quickly and they can start their masterpieces all over.
It's nothing fancy, but it will keep the kids busy for a few minutes so maybe you can at least go to the bathroom without someone banging on the door. =)
You can find this awesome post linked up to the following Fantastic Parties: