Sliding glass door lock status: I've mounted contact sensors on three sliding glass doors to tell if they are locked or not. The sensor is mounted on the glass and the magnet is on the locking lever. When the lever is in the lock position the contact closes. When that and a second contact sensor on the door tells me the door is closed, I know the door is closed/locked.
Cooktop safety: I use contact sensors hidden in my cooktop that tell me if any of my stove burners are on - if any are on an LED strip at the cabinet baseboards lights up red. When any burnder is turned on it also turns on the under-cabinet lights so we have task lighting.
Outside Electrical panel: I have a contact sensor on my electrical panel to remind me if I leave it open (HE also notifies me when it is initially opened).
Fridge/Freezer door & temp: Contact sensors on fridge and freezer doors to remind me if they are open too long...not really creative, but important. Contact sensors (and I also use a button in one case) in fridge and freezers to monitor temps. (I had some extra contact sensors that do temp, so using them in the freezer allowed me to make use of them.)