It borders on being a competition for some -- crystal, a chandelier, silver -- and as much French food as possible. That said, for most, it's just whatever the folks feel like. I've taken poached salmon with dill sauce and I've taken Thai carry out. Foodstuffs borders on being the "official" place for folks along the lake to buy their picnic food, if they aren't putting together their own baskets. Lots of take-away options offered:
http://www.foodstuffs.com/menus/meals-to-go-menu.php. They might be good for inspiration.
I tried once doing the posh route -- china, crystal, etc. -- and decided after one time that I'm not remotely interested in every carrying anything that heavy again, especially since you have to carry it all out again.
Depending on the number of folks, roast chicken is always nice (doesn't get soggy like fried can). Potluck last year, for a group of a dozen, we had lots of finger food, cold salads, and hot casserole type dishes. (I made a dish of quinoa, bacon, and wild mushrooms.) Someone brought brats and someone else brought pastry. The main focus was on savory, but there were a couple of sweets -- and plenty of wine. Water is a good idea, too, because wine is nice but doesn't quench your thirst. They do sell food at Ravinia,too -- and have a nice ice cream kiosk.
So seriously, it's anything you want.