I bring stuff to fix a flat - tire irons, pump, spare tube, patch kit - and a multi-tool for anything else that might need tightening, fixing, etc. My multi-tool has a chain tool and screw drivers. The chain tool is something I've never needed but I kind of like having it on the tool. Screwdrivers come in handy for adjusting the limiting screws on derailleurs. The Allen wrenches on the tool are often used for adjusting things, and also for tightening bolts which may be loose.

I have a lightweight Blackburn rack on the back of my daily ride. I put a rack trunk on it. I carry the tools in it, plus my wallet, sunscreen, energy bars, a sandwich, a book, a hat, a shirt, etc. I'm afraid it makes me look a bit Fred-ish, but I really don't care.