Long standby shift at a hotel (think Hilton style). My employer (not the hotel) had a storage room that was just an old hotel room with all the fittings taken out- No bed or couch, just storage racks. I got super bored, took a nap in the old bathtub.

The staff bathroom of an abandoned diner on top of a mountain in Japan. I was cycle touring, didn't want to put up the tent if I could just go inside. The big windows and proximity to the road meant I didn't want to be using a flashlight inside, so I went into the staff bathroom, no windows. Also no toilets or anything, just a bare tiled room. Weird place to sleep, but I went into the main area to make breakfast, it was an amazing view. Bonus for not getting the tent wet.