At a minimum you'd want tube/tires/patchkit/pump or co2 if you prefered. The allen wrench(s) the fits the bolts that would disable you if they loosened a few zip ties and maybe a master link for your chain in case it breaks (it weight next to nothing). Oh
Cell Phone!, ID, CC, Cash?
On my giro right now
Cell phone on the stem
Organized into categorized zip lock bags:
Multi tool with torx bit (I have disc brakes)
6" of chain and a 2 master links.
Adjustable wrench
Electrical tape, Zip Ties (2 sizes), Velcro straps
Handiwipes, Latex Gloves
6 spare AAs
2 spare AAAs in case my speaker amp goes dead
A 2AA to USB charger for my phone/mp3 player
Pump (topeak road morph)
Glueless (park is the only one I have found that works well) and Glued patch kit
Folding tire
2 spare tubes for my tires
3 Tire levers
Quik Stik tire changer (jury is still out but I'm trying it)
1 spare tube in 700c (must have been carrying for my wife)
-Soon to be 20" tube for my wifes new bike
Schreader/Presta adapter in case my pump goes and I have to use a schreader.
2 fiberfix spokes
A couple fruit leathers and granola bars (save stopping at a store for a little something)
Sun Lotion, lip balm
Local maps
Few $ Cash
Credit Card
ID
On my xtracycle I carried a spare rear wheel with tire already mounted for a few weeks. I did it mostly to try to get a reaction out of my old director but somewhat because I just put a new wheel on and had been having alot of problems with heavy loads and breaking them. 100lbs+ of dog food and things like that.