Well, that would depend on who is in the passenger seat (my mother in law for example)
1. The keys (so that I could automatically lock the doors once I've jumped out)
2. Fire extinguisher (she might try to use it)
3. My cell phone (to take happy snaps while she screams, fries and sizzles).
If I go for a long drive I wrap a chicken in foil and put it in the engine bay.
I find 2 hours at 3000 RPM cooks it perfectly.
So I would grab that and have a chomp while I waited for the fire brigade.