I carry one of two bags. In my backpack is (off the top of my head:
- flashlight
- spare battery for flashlight
- computer mouse
- laptop riser (to keep it cool)
- iPod
- earbuds
- another set of earbuds
- checkbook
- misc. pens and pencils
- some decorative buttons
- laptop and power supply
- a Popular Science magazine
- misc. connective wires for the laptop to other devices
- business cards (mine and others)
- a paperback book I can't recall the title of
- an epi-pen

in the other bag 9the one I take to work daily):
- 4 bottles of water (2 liters total)
- epi-pen
- rain-proof windbreaker (30% chance of rain today)
- asthma inhalor
- a copy of _The_Subtle_Knife_ I'm re-reading
- chapstick