Lately all I carry with me is:

· Wallet
· iPhone
· Bluetooth + headphones in little zipper pouch
· makeup in bigger zipper pouch
· Holga or Olympus Stylus or Nikon D3100
· sunglasses wrapped in bandana (bandana doubles as snot rag)
· & keys on carabiner on jeans

I haven't been biking much since it's so cold out, hence the pared down list. When I do I break out the tool kit, u lock, and extra clothes.

I think an extra cog is very useful. I carry 8 extras, plus another chainring.