Here is a start:
Map / GPS / Compass
Flashlight / Headlamp (LED is good due to awesome run time)
Food (think high calorie, long lasting)
Extra Clothes (layer system, insulation and shell)
Sunglasses / Sunscreen
First-Aid Kit / Medicines
Pocket Knife / Multi-tool
Waterproof Matches / Lighter / Firestarter (think redundant)
Water / Filter / Bottles
Whistle / Mirror
Insect Repellents / Netting (environment dependant item)
Cash
And I disagree completely with this:
"I would avoid tactical-looking packs in favor of a plain daypack. A JanSport-like daypack will attact less attention if you're using your pack during a serious social breakdown. Tactical packs can make you a target, as you'll appear to be very well-equipped, possibly with a firearm. My $0.02."
The best pack style will be 1000+cu.in. and if you are carrying a heavy load will have a real waist belt that can support some of the load. Check out the Camelback line of packs, they are very good with built in water. Also some of the Blackhawk stuff is good too.
I like the larger "hydration" style packs with the built in water. Very convenient set up.
If you are doing this on a tight budget, then yes, buy some cheap daypack from Big 5. Better than nothing.