Most of the instrument-related stuff- picks, capos, spare strings etc lives in the cases so it's there if I'm just taking out one two instruments to a session I'm covered for minor emergencies. Any time I'm doing any serious playing I take my emergency toolkit which contains an assortment of screwdrivers, various tools for unsticking and tuning harmonica reeds, spare bridge pins & bridge pin remover, plus a nail file in case of breakages. Every single one of these things has saved a gig at least once!

For plugged-in gigs I have a big silver case with stuff like leads, spare batteries + power supply, various preamps, EQ & switching pedals, plus a spare DI boxes for those occasions where our 4-piece band with 7 instruments between us turns up to find a couple of mics and a DI box because someone didn't know the difference between "acoustic" and "solo singer/guitarist". ;-)