I just got a Guard three trumpet case (even though I have one trumpet). I am working on having the following in it:
Mutes (may put them in a mute bag then the bag in the case)
Small but powerful flashlight
A few small key chain/pen lights
Mouthpiece (in it's own bag)
Some basic medical supplies such as bandages, Advil, Neosporin, etc
Swiss Army Knife
Screwdriver and small wrench (in case a stand is wobbly)
If I can fit it, a collapsible full music stand (never know if there will be enough stands at a performance. Trust me this has happened to me a lot.)
Small travel toothbrush and toothpaste (doesn't hurt to brush and rinse twice before a performance)
A sheet of the key signatures (I still can't memorize them all)
If I have to march gloves and black socks. Actually I should carry extra gloves and socks. Never know if someone will forget one of them.
Basically I have a small survival kit.