I've seen a lot of CO2 inflators fail. I carry a compact two-stage pump, and mine fits next to one of my bottle cages.

The Topeak tools have a great reputation, and if you carry one (or another folding hex wrench tool) you wouldn't need to carry everything else suggested.

I always have my keyring with me, which includes a 1xAAA flashlight and a whistle. I don't go out without ID, a credit card and (at the minimum) a $20 bill. Water is a given; I tend to bring enough to share.