Thanks for all of the replies. The brownie mix my wife was going to use was made in a factory that may have exposed it to peanuts. Since one of the young people in the group is allergic to nuts, she wasn't able to make them. (of course, as Murphy must have planned it, the young person didn't show up anyway). On the other hand, the grill was in use for almost the entire time doing hamburgers and hotdogs so marshmallows probably wouldn't have been possible anyway.
So we copped out and brought big bags of Chex Mix and Cheese Puff balls (For anyone who is unfamiliar with those, they are similar to these: http://www.walmart.com/ip/UTZ-Cheese-Balls-28-oz/37319722
I do appreciate the options I saw here.