The original question is tough. I originally thought "man bag", but not with a tux! (A nice man bag with a suit might be OK.)
With a tux, I'd bring minimal gear:
- thin pocket knife
- fauxton flashlight
- car key (not full set of keys)
- cell phone
- money clip with credit card, ID, etc.
- some business cards
- square pocket (doubling as handkerchief)
- sunglasses
- 2 or 3 bandaids (Why not? They use no room.)
- camera (if cell phone camera is insufficient)