This Project is meant for Eagle scouts.
Decide for yourself if it is appropriate for your younger scouts or not.

Work with your local fire department to find out if they like this idea. Also find out what requirements they have for painting of hydrants.
Map out locations of all fire hydrants.
Convince local citizens to adopt a fire hydrant close to their home. They take responsibility to keep it painted, clear of debris, and clear of snow in winter. Depending on local regulations, they may customize the painting done.