My seat bag contains: patch kit, tire levers, small folding bike multi-tool thingy (given to me by a friend of the family when he got a fancier one), a $10 bill, a couple of bus tickets, a few bandaids, and some tiny packs of neosporin appropriated from the first aid kit at work. There's space for my keys, but it's rare for me to actually put them in there.