SO's backpack
He's been unemployed for a few months and started riding with me to work, and I don't complain because he's a very handy mechanic lol.
But for those rare days I ride alone I keep my tools in my panniers along with all my stuff (my kit consists of: spare tube, pump, tire levers, patches). In my early cycling days used to have a seat bag, which was kinda small but was enough for a spare tube + well hidden spare lock key (in case I forgot my house keys)