Surely you only really NEED a bass?
IME, you should have:
Bass
Amp
Tuner*
Ear plugs!!!!!!
2 x Every cable you need
Strap
Spare strings
Picks
All the screwdrivers, allen keys and tools you need for your basses
Torch*
Fuses
Tabs/Chords/Notation/Lyrics*
Contact details to get more bookings*
All of the * can conceivably be done with a phone these days. I also find them invaluable for googling manuals for confusing house mixers, taking pictures of chicks at shows, playing music during breaks, etc etc.