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)

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If you're reading this, it's too late.