The Costume Kit
Primary: full costume with all pieces accounted for and tested. Repair supplies: safety pins in multiple sizes, fabric tape (both iron-on and pressure-sensitive), hot glue gun with extra sticks, a small amount of the main fabric from your costume, needle and thread in matching colours, super glue, E6000 adhesive. Spare parts: an extra set of any small parts likely to detach — clasps, buckles, buttons, snaps.
The Personal Kit
Comfort: moleskin for blister prevention, bandages, pain relief, any personal medications. Hydration and food: a refillable water bottle (convention centres are vast and dehydrating), energy bars, cash for food. Navigation: downloaded convention map, schedule highlights marked, hotel key card in a costume-accessible pocket.
The Photo Kit
If you're serious about convention photography: a small mirror for on-the-go hair and makeup checks, touch-up makeup in your bag, a second phone battery or charging bank, and a clear sense of which shots you want and where in the convention to get them. The best convention photography happens in the first two hours of each day before costumes accumulate damage.
What to Leave Behind
Anything irreplaceable or expensive that isn't part of the costume. Anything that won't survive a full day on a convention floor. Your second-best version of any prop — conventions are hard on props and the one you bring will take damage.