Tour around your neighborhood and you'll notice that while many people make do with plastic mailboxes nowadays, there are still the traditionalists who use basic metal ones, and these can begin to look a little dull and beaten up as time passes.
As a group you could go door-to-door offering to paint mailboxes or designate a specific day (through flyers and posters) where you'll all turn up ready to spruce up the mailbox of anyone who requests it. You're going to need paint specially designed to adhere to metal and that's able to stand up to the elements. Start with basic colors (black, white, red) and then add a few more to the mix (navy blue, green, yellow).
Of course you should give homeowners the chance to select a color of their choice — remember you can mix different colored paints together to produce a new one if anyone has a special request. You're going to need paintbrushes and some small drop cloths to make sure you don't get paint on yards or curbs as you go past. The best time to do this job is on a Saturday afternoon — the mail has just been delivered and the boxes have all day Sunday to dry before they are likely to be touched again.
Before you set out, decide whether you want to charge a set amount for each mailbox or simply ask homeowners for donations. You might even want to charge something extra to add stenciled patterns to the boxes.
Each week you can work your way from neighborhood to neighborhood, and before you know it you'll have earned a substantial amount of cash for your chosen cause or charity.