Spare strings (usually complete set and extra B and G), tuner, capo, cleaning cloth, Altoids tin with 4 or 5 different picks and 2 or 3 bridge pins, 2 or 3 spare 2032 batteries.
Outdoor festival - add hankerchief, Tums, Tylenol, often spare tuner and spare capo.
Separate bag for gigs, 3 ring binder with tunes, music stand, instrument stand, clips to hold the sheet music in place, microphone, 1/4" cable. Normally now gig on mandolin so no need for microphone or cable. Unless I'm carrying a portable PA system, then include microphone stand and cable.