I'm afraid I don't currently have a cosplay that could be considered casual and comfortable. So I'll just be in "civvies." Naturally, it will involve an otaku-ish shirt of some sort, because I can.
I have a black newsboy cap I've been fond of lately. If it helps, I'll wear it.
Oh, and a notion: unless we are responsible for our own eating utensils, we will probably need paper plates, napkins, cups, and plasticware. Is anyone going to bring those?