The bare minimum I would carry for a long ride...
(1) Tire lever
(1) 12g CO2
(1) Patch kit
Cell phone
ID, credit card, health insurance card
House key
(1) 6oz Gel flask
(1) Water bottle

However I usually carry an extra tire lever, 16g CO2, and water bottle. I'll also carry a spare tube or two. That way I have enough water and tools to get my through a few flats. Two 16g CO2 can probably fill up three 700x23 tires, but probably at a lower psi.

I don't ever carry any tools though unless I go mountain biking. I'm a pretty good mechanic, so I keep my bike in good shape. If something more than a flat were to happen, I probably got unlucky.

Oh right I forgot to add... carrying bills is useful. You can use them to boot stuff or buy from places that don't take credit.