A camera WITH a strap: If you want to grab photos of the impressive cosplays, celebrities, and cool displays you'll see all over the con, bring a camera along — but don't forget a strap for easy carrying, or you'll regret your photography hobby faster than it takes Hall H to fill up. (For the newbies, that means fast!)
Sunglasses: At some point or another, you're going to get stuck outside in a massive mob of con attendees. Help yourself hate life less by bringing sunglasses, so at least you're not blinded and can check out the cool merch purchases and cosplays around you.
Airborne and hand sanitizer: The large crowds at conventions are part of the fun, but they also mean you're getting exposed to a whole bunch of grossness. Keep your immunities up and germs off with your own arsenal of protective goodies.
A marker pen: Whether you're hoping to get a celebrity autograph or want the original artist to sign your comic book, it's always good to have a Sharpie around at cons.