This girl scout troop helped the local Parks and Rec hang signs around local parks that say “Young Lungs at Play,” to discourage smoking near the playgrounds. The scouts spray painted small pans with green paint, added the vinyl sticker (you can have custom vinyl stickers made online from many sites for just a few bucks), and screwed them onto the wood equipment and signs. If your local playground is metal or plastic, you can skip the pans and screws and just stick the stickers on (you could still teach them how to use a level and chalk to make the stickers level if you want to teach a skill).
If you want to do this activity, be sure to get permission from Parks and Rec (or the HOA or whoever is in charge of the park), and include a little lesson on the Word of Wisdom, too!