I, like most people, forget to change out my air conditioning filter as often as I should. The ones that I buy should be changed out monthly, but, I probably do it 3 times a year, because I forget.
How about designing a mock-up flyer and go to a few ritzy neighborhoods and doorhang? What if you could charge $15 a month to come out to do it? I know that I'd pay it.
The filters cost around $4.88 at Wal-Mart, and you could probably get them cheaper in bulk.
Of course, there are logistics questions - times, are there people home, do you get a key, etc. However, If you had a neighborhood with 100 homes, and you could get 50 of them, and you make $10 off of each one, and you can do an average house in 5 minutes, you could make $500 a day.
What do you think about that?