For super grab-n-go, as in, 5 minutes....
A small back-pack and a canteen.
Water purification tabs, maybe a little bottle of bleach (for water purification), and some nuts, sunflower seeds, beef jerky, and floss. I can't stand when beef jerkey gets stuck in my teeth.
A pocketknife with bottle opener to open up beer, and some extra underwear and socks. Maybe one of those travel pillows since I would probably be sleeping in my car.
I'm assuming when the zombocalypse comes I can hit up convenience stores for grab-n-goes to get food. If I have my car, I would rather sleep in there than outside. Even if I run out of gas, it's at least decent spot to sleep. Could be trouble if zombies swarm the thing while I sleep, but hopefully I would have fellow zombie squad members to take turns doing watch. I mean, we can only mitigate zombie threats so much.
Basically, I think keeping some water and canned food in the trunk isn't a bad idea. Not a months supply mind you, but enough for a few days. Same with the storm cellar. 3 days worth should be more than enough. Most tornados don't last that long.
I normally just try to keep some bottles of water and a lunchbox full of canned food and can opener in my car. Then I don't have to worry about "did I grab something or not grab something."
As for important documents, keep a fireproof/waterproof safe in the house, and keep copies in a safe deposit box in a bank (electronically via a flash drive or physically). My bank is about 35 miles away, so if this event is so catastrophic it takes out my bank and my house - Then it will probably get me too.
From my observations of natural disasters (katrina) and zombie outbreaks (Romero's Dawn of the Dead) - the best balance is to make sure you have some groceries at home, some stuff in your car, and the ability to go get more if need be. It's kind of diminishing returns to go excessive on preparation - there is some value to KISS. If something lasts more than three days, your survival knowledge is going to be worth more than what you have on hand.
And what is the benefit of really bugging out unless you are just going to stay in a hotel in another state to escape a hurricane or civil unrest? I mean, in a zombocalypse scenario, your house may be a great fortified position, and being in close proximity to now abandoned convenience stores could be useful. Pepsi and snickers have an incredibly long shelf life. As long as you can fend off those pesky zombies.... and as anyone who has played Nazi Zombies on X-box knows, this is where teamwork and cooperation is essential.
In fact, if you could come across without scaring your neighbors.... it wouldn't be a bad idea for communities to make plans together. Heh, you could be a community organizer!
You know, everyone keeps 3 days worth of water stored away ($3 at Wal-mart) and a weeks worth of Toilet Paper. If there is a zombocalypse, we look out for each other's stuff.... and even if they aren't gun owners, the extra coordination could be very helpful. And zombies, or thugs, are vulnerable to prepared and vigilant citizens with baseball bats. Everyone needs to sleep, but you could take turns with your neighbors keeping a neighborhood watch, and if a gang of zombies or looters is approaching, they could wake everyone else up.
*Insert thug for zombie where appropriate, and realize that the zombocalypse is a parallel to a disaster where there is great social unrest, eg. katrina.
Most people on THR are individualists (I mean this in the positive way), they are prepared to do take care of themselves. That's a good quality. But the fact is, communities can survive disasters better if they are united and have a plan. I mean, I can only shoot so many zombies before I need to reload, and this is where my fellow zombie survivors can be useful. I like my sleep, and I would feel better sleeping in a zombocalypse knowing someone was helping to keep watch while I did so.
I guess kind of off topic, but related to the whole narrowing down the BOB. With convenience stores every 10 miles where I live, it's hard to make the case that I need a crapload of stuff.