Bass (x2 if it's a long-distance or expensive corporate affair)
Rig
Peripherals (wireless, FX, etc)
Leads, includes plenty of spares
Stands
Toolbox (contains strings, tools, gas solder iron, superglue, electronic bits, fuses, multimeter, lead tester, nail clippers, you name it, it's in there!)
Bottle of drinking water (if I fill it at home, at least I know where it's been..!)
Towel
Change of clothes
EARPLUGS