Small metal water bottle (with carabiner clip, and sipper tip, can always refill this)
Mainstay Ration Bars (3 days worth)
Water Purification Straw (check the Clear Sip threads here, it's a damn good filter straw for the money)
Two-way radio (you should have one also)
Crank flashlight/radio/charger (small and light, and can charge your cell phone, give light, and AM/FM radio)
Small first aid kit (like the kind you get for $7 at WalMart, about 4 inches by 4 inches)
Altoids tin worth of any needed or desired pills/meds (including a few multi-vitamins)
Small laminated map of area (showing key landmarks, possible supply locations, etc. Scout it out, indicate on map).
.22 pistol and 50 rounds
Whistle (around neck)
Firestarter stick, waterproof/windproof matches, Zippo lighter
Extendable Camping Fork (for roasting something over a fire)
Inflatable Pillow (for comfort)
Change of Socks and Underwear
Deck of Cards showing Edible/Poisonous plants for your region
http://www.campingsurvival.com/playingcards.html
Pretty much covers Water, Food, Fire, Protection, and Communication, and should still leave weight for some good stuff like hard candy, beef jerky, etc.
If you are somewhere where it can get cold, some additional items
Emergency Thermal Blanket (not the cheap $1 kind, but more around $10)
Pocket Warmer packs
Some Thermal Underwear (stored with the BOB, not in it)
Also, not everything has to be in the BOB. Consider a change of clothes and shoes to store with the BOB. You don't want to bug out in a business suit. (and you can store things in pockets to reduce your load weight).