My favorite past-time is to take a picture of "distressed" properties if I happen to come across one. I drive a lot during my work shift and I see a lot of houses that obviously don't have garbage service, or are vacant and overgrown, or just simply unsightly. I organize my photos in folders according to areas... NE, SE, North, and so on.
I'll hold onto the photos indefinitely, for my desire is to eventually see the property fixed up and sold. That's when I go into action. Whenever I see one of these properties sold or cleaned up, I'll pull up the appropriate photo and get my pen ready.
I'll print a copy of the photo of, say, bags of garbage on the driveway and write the new owners a letter. The letter will say something like: "Forgive my intrusion, but I just wanted you to know what your house used to look like. I know you weren't responsible for the bags of garbage on the driveway in the included photo, but it's always nice to have some perspective. Look at the photo of the bags of garbage and just think about what once was, and what might be again if you're not careful. I wish you the very best in your new home. Sincerely, A Concerned Citizen"
I never provide a return address, just a letter with a photo. I like to think that it offers some joy to the new owners, like they're having a part in making our city better. Some donate money to charities, other volunteer their time at shelters or food banks. Me? I send complete strangers years-old photos that show how their new home used to be a nuisance property.