Quick fixes? Depends on how quick you are

Assuming you've isolated the creak to the BB, it's possible that some dirt got into the threads or that they are a bit dry. The fix in this case would be to remove cranks & BB, clean/grease threads and reinstall. Maybe 30 minutes if you know what you're doing, much longer if not.

If the BB feels solid otherwise and doesn't behave as though on its last leg, this could wait until you have some time.

IIRC, this is a bike you purchased secondhand. If the bike has some real miles on it, you might just install a new BB cartridge. Only $20, and you've already got it torn down.