What to take?

I carry on all rides: 2 levers, an inner tube, a patch kit, CO2 inflator w/ 2 or 3 cartridges, a tire boot, cell phone, multi-tool, drivers license and CC card, Road ID bracelet. Everything with the exception of the bracelet is in a seat bag.

I use a backpack, but mostly because I carry my water on my back. I have spare parts and tools in a seat bag, but my backpack is like a little disaster kit. I carry a spare pair of pants (because I am one of those people who has the luck that I'd be half way through a ride and have a clothing blowout), a six pack of Gatorade, some energy bars or at least peanuts, a 100oz bladder of water (don't laugh, all the liquid is gone by the time I finish... I'm a sweater), a cell phone, and pepper spray (we've had some attacks on cyclists on the trail I usually ride, and to be fair the pepper spray is usually stuck to the bike unless the strap gets wet).