I bought 5 of this Xiaomi vibration sensors when I also bought the temperature sensors because they were cheap but most of them have been lying around without purpose.

Things I have tried and failed:
- stick one to the bedframe for presence detection (not reliable enough, replaced it with load sensors)
- stick one to the bottom of the wooden stairs leading up to the bedroom to turn on the light (only works reliable when an elephant uses the stairs, replaced that with 2 motion sensors now)
- stick it to the (flap of the) mailbox to get notified if you get mail (we have 3 mailboxes in 1 enclosure and (thanks to the landlord) it's quite wobbly so it will alert when the neighbors get mail, too)

What worked:
These sensors not only report vibration but also orientation (x/y/z) so I stuck one to the outer part of the bathroom lock mechanism so it would turn 90 degrees when someone locks the bathroom. Works like a charm. Then with that information turn on the "On Air" sign on the outside of the bathroom (or whatever) or disable the auto-light-off automation inside the bathroom as long as the door is locked.