Brisket by nature is pretty casual, but a few suggestions come to mind. Gulliver's Corn from Hammster (Mike) on the BB is a great make-ahead for a crowd. Chunky Spiced Applesauce with Cranberries (CL) is wonderful; another make-ahead. So far, with those, you'd have dark brown brisket, yellow corn, & pale red applesauce w/ cranberries. Now you need green . . . I don't have as many make-aheads, in green, for a large crowd, but I'll give it some thought. Or maybe orange--CL's Carrot Souffle is terrific, and I think I've posted it with my make-ahead hints for the BB (it's actually quite easy to make).
Will people be seated or standing to eat? CL's 12/08 Hot Chocolate Fudge Cakes for dessert are make-ahead & wonderful, rather impressive-looking but very easy, and only suited to sit-down eating.
For an app, I love Ina Garten's Guacamole (Food Network) more than any other guac I've ever had (and that's saying something!).
If you're afraid of butter, use cream. ~~ Julia Child
As you cook, you enjoy omniscience about food that no amount of label reading can match. Having retaken control of the meal from the food scientists, you know exactly what is in it. (Unless you start w/cream of mushroom soup, in which case all bets are off.) To reclaim control over one's food, to take it back from industry & science, is no small thing; indeed, in our time, cooking from scratch qualifies as subversive. ~~ Michael Pollan