A wristwatch: Fumbling around for your phone while you rush between events is a hassle, so just wear a watch. Plus, if you have a smartwatch, you'll see how much you walked over the course of the convention — last year, I averaged 12 miles per Comic-Con event day.
A sewing kit: If you want to be every cosplayer's BFF, bring along a sewing kit for quick repairs. You would not believe the amount of rips and tears that come with the territory of wearing an intricate costume in a packed convention hall!
A sanctioned meeting place with your group: Not everyone in your group is going to be as prepared as you, which means dead phone batteries. Always have a designated meeting spot in case you can't get in touch with one another!
The event's own app: Most cons have their own app, which means you can access schedules, maps, friendly reminders, and the like all from your phone. This can be a lifesaver if you get lost!
A sense of respect: OK, so this isn't an item to pack, but it's vital that you treat fellow con-goers — especially cosplayers, who are unfortunately touched or photographed without consent because of what they're wearing — with respect. Enjoy their costumes without grabbing or harassing them; it'll create a better con experience for all involved!