I always bring the usual travel stuff and then add in the robotics related stuff. I suggest bringing a camera, comfy shoes to walk in, hair/face paint, cards or something to do in the .3 seconds of downtime, stuff to trade (buttons, keychains, banners, shirts, members...
) and anything else you can use. Also I always bring my laptop just incase I need to do something important and to do scouting analysis.
Also depending on how you're getting down, I'd bring extra snacks and drinks. You can't take them into the Dome but, I think you can bring them into the GWCC (I always do) and also it's nice to have them in the hotel. My mom and I bought 1 case of water, a case of monster (like $4 a can down there), a case of Shasta soda, and a big assortment of chips for the team.
And last but not least, bring your patience and best behavior. Many of the Vets will tell you (especially drivers) that everything at the Championship will run a little faster. There is some damage to the GWCC so we will have ~1/2 the space as usual so traffic will be tough and everyone will be tight. Also be prepared to leave for your matches about 15 minutes early or more because of the walk from the GWCC to the Dome.