Pump, tube, and just-in-case patch kit.

On one bike I have a Topeak Road Morph G, on the other, a Zefal frame pump. Either one pumps up so easily that if the floor pump is in the basement, it's not worth walking down the stairs to get it when I top off before a ride. Pumps seem easy and cheap to me, so I've never been tempted by CO2--don't really need another thing to spend money on and keep track of.

Also, tweezers. They are the wonder tool for extracting that sliver of glass or wire from your tire.

I'm curious if there is a gender difference on pump vs. CO2--personally, I'm not particularly eager to be biking alone in the boonies 50 miles from home, and worrying about getting one more flat than I have CO2 (what would that be? F = C+1?) But as a woman, I also grew up being told not to do something as idiotic as walk down a street alone at night, much less get stuck with a flat alone in the middle of nowhere.

Or maybe it's just my slightly neurotic/OCDness that likes to vividly imagine worst case scenarios, and tell me "I told you so" if they come to pass, and has nothing to do with gender.