This one isn’t too hard. It’s also pretty cool, and surprisingly useful.
This is one we found in a Summer Reading Packet. You can also find it online. Here’s a good site: http://fun.familyeducation.com/childrens-art-activities/crafts/29578.html.
The chalk does take awhile to dry, especially if you use toilet paper rolls. Also the cardboard on those sticks to the chalk, which is kinda icky. Try other molds, the ice-cube tray sounds like they’d turn out well.
After they are done, you could hold a sidewalk art competition. Or use them to promote upcoming activities.