One spare tube, a patch kit, and a road morph mini. The pump clips under the bottle cage and is barely visible.
I see some here insist that co2 is not a problem, and perhaps for them it isn't. But many is the time I've stopped to help people who blew their co2 into the air and is now stranded.

I figure I'm good for 5 or 6 flats with my system. Not that that's happened more than once.