I always carry duct tape - it has so many uses. And also a little bit of 2-sided tape. I just put a couple of turns of each tape around a small piece of PVC pipe and put that in my bag. Also a tiny tube of sunscreen. I never could master the use of a frame pump (please don't laugh) so I carry one of those CO2 cartridge things to pump up my tires if I flat. They work pretty well. And finally a $20 bill for emergencies. And that's it! I try to carry as little as possible so my bike bag is pretty small.
