If you work with quick release systems on all of your cameras, a washer can make a great tightening tool. There are plenty of great multi-tools out there, but you might forget it one day when you're on a shoot, or you may not feel like bulking up your pockets too much. In this video, youtuber thefrugalfilmmaker suggests you attach a small washer to your keychain for tightening and loosening your quick release plates on your cameras. It's small (just the right fit), lightweight, sturdy, and works well. The other keychain tools he recommends are pretty good ideas too.
Tip: Keychain tools [YouTube]