Yeah, don't forget snacks and water. This year was my first AX and there's so many cosplayers to photograph that I ended up not eating lunch, so the snacks and water are very helpful.
Depending on your style, you don't even need a flash. Some photogs prefer natural light and do amazing work with just their camera. But if you do bring a flash, Access's suggestion of making your own diffuser/bounce is easy and cheap and customizable. I'm lazy so I just buy them lol. For hall shots, the ceiling at LA convention center is really high and you're not usually near walls so there's not much to naturally bounce off of unless you can pull the cosplayer off to the side.
Me personally, walking around hallway shots = I bring just my camera + one flash with a small sling bag holding the batteries/memory cards/food/water. For shoots, I bring couple light stands and either leave them bare or use umbrellas/softboxes depending on what I'm doing.
You can call me Jono
San Diego Comic-Con, Otakon, Dragon*Con