I carry/have:
in seat bag:
allen wrenchs (park)
spare tube
generic multi-tool
messenger bag:
shirt, socks
cereal bar
pump
wallet
cellphone
lock(s)
keys
on bike:
rear blinkie (red, only for early morning, sunset, i don't ride at night)
water (!!!)
cyclocomputer
and i wear gloves and a helmet. and i'm thinking about getting a mirror soon. jeez. i thought it was pretty minimal till i listed it all out. guess not...