How about this one?
Laundry machines that send a message to your phone when they're done (or in the event of a load imbalance, or other anomaly).
Not so much for home; that'd be more of an IoT thing, with status available through the household network. For laundromats, guest laundry at motels, and such.
Maybe use NFC to send the phone number. (If done right, this could discourage pranking.) Better still, combine with pay-by-bonk.
(Not that I use pay-by-bonk, nor even have it enabled. I haven't even looked at Google Wallet lately, aside from noting that, when I set it up years ago, I seem to have used a credit card which is now long defunct. Is there even a way to set it up with a modest cash balance and no associated credit card?)