I'd take the one of the coolest things about tf2, the demoman's sticky bombs and use it during the one crappiest times I could think of - like traffic! Seriously, some jerk cuts you off in the middle of an intersection what would you do? Why sticky bomb the back of his car! Hope you get 2 or 3 crits in that new pile of spiky death. Wait for the poor shmuck to go around a corner and then KABOOOOOM!
Ah... or the rocket launcher! What better way to speed up the grocery line than to whip that thing out. Now of course not everybody would immediately know what it was and what they should do, so you'd have to blow the smithereens out of the soda cooler. I'm sure the soda cooler would take 2 or 3 hits just like a dispenser. Maybe one shot would be a crit and you'd get a multikill. Hopefully your purchase is good enough for a last meal, for I don't think the local law enforcement would be too concerned with your well being after that point... just make sure you eat what you want BEFORE you go to the car.