Helmet, sunglasses and gloves are a no-brainer. The bag on the back carries a spare tube, the tire changing tool, the cylinder of CO2 (whatever it is) to inflate the tire, $20 cash, my debit card (in case I'm really a mess, and need to call a cab), and my cell phone. I also have all the water I can carry (I'd rather have a little more weight than run out of water), and some fig newtons (I'm converted!).

BTW, while I know HOW to change my tire, since I always ride with DH, I don't ever actually do it. Why should I change my tire, when I've got a perfectly good spouse to do it for me?

Oh yeah, and I carry single-package Wet Ones (or whatever the 'moist towelette' is called). Good for cleaning the hands after changing the tire (well, okay, for DH to clean HIS hands when he changes the tire! ), or for other emergencies. For long rides, I also carry some Tylenol, some more serious drugs for DH, and any, erm, girly things I might need.

It sounds like a lot, but it all manages to get packed into a pretty small space.