In my bag (A Lowepro camera bag) I keep:
Top compartment:
Spare AA batteries for the GPS (Garmin Oregon - it eats batteries, so always have some charging!)
Food (am always hungry! - fruit, carrot sticks, the odd flapjack)
Bottle of water
Scarf (I always carry it even in summer!)
Keys
Phone
Purse
Chopsticks (odd, I know, but if I have to eat something and my hands are dirty, these do the trick)
PDA (although it is kind of obsolete now we have the Oregon and the battery on the PDA is a bit pants )
Spare camera battery
Zippered outside pockets:
Left side - a couple of pens, some bouncy balls and random other swag
Right side - 'Thank you for not driving' stickers, a small torch.
Bottom compartment:
Camera (Canon EOS 350D)
Geocaching gloves of protection +2
Many bouncy balls
Plastic Frogs
Plastic Bats
A few toy cars
Any TBS
A toy dinosaur
Spare paper
.....I think that that is about it...it doesnt weigh much really!