When I was in high school I volunteered to help with VBS at my church. This activity is engraved in my brain because it was so much fun! As you can see, some 20 years later, it still works!
Fill a bucket or bowl with water and give them a household paintbrush. They will love painting the driveway, rocks, or brick. They watch the colors change simply because it is wet. You may need to set your bounders on what gets "painted" but the kids are sure to have fun and get a little "paint" on themselves as well. Enjoy!