When i get invited to a bar-b-q, my favorite things to bring are portobello mushrooms and corn on the cob. They're easy to deal with. Marinate the portobellos in balsamic vinegar for an hour beforehand and then throw'em on the grill like burgers. I remove the stem and cut slits on the cap. In a group of non-vegetarians, the sight of 5" diameter mushrooms on the grill is a conversation-starter.