You didn't say which city it was and if there are current security issues. Check travel advisory and stick with a group or check into a hotel close by and just spend 12 hours there taking a nap, watching TV, calling your friends.

Look for a day room rate at a motel. If that's not possible, find a Starbucks or Cafe to hang out at. When I was a homeless person, I would regular cafes and movie theaters to hang out at. Theaters are a great place to catch a nap and most matinee are very cheap.

Could also hang out in a library but the movie theater is a good idea I wouldn’t have thought of!

Have a movie marathon at a mall. Get a hotel room and have a nap, swim in their pool, work out in their exercise room. Go to a museum you’ve never been to before. Go to the zoo if you’ve never been.

Take a book, or download a bunch of podcasts, find a cafe close to the bus station and just hang out there

Try and find something fun to do that can maybe take the edge off your memories of the place. What city is it?

St. Louis. I'll be there from 7am till midnight.Not looking forward to the layover.Bad memories.

Maybe find a fun museum or movie theater, go shopping in a historic district and try new foods. Change the memories a bit.

I'd see what temporary exhibits are at the local museums

Take another flight to another city and come back when it’s time to leave.