I like the ideas of having people bring a covered dish, side dish or what have you. We do that at family reunions, but anyone who travels a distance isn't expected to have to bring anything. The BBQ or picnic thing sounds great too. You can get family size packages of hamburgers or hotdogs at many of the supermarkets (around here we have Redner's and Giant that offer good deals). There are always family members that are known for their macaroni salad or baked beans, they might even feel honored that you asked them to make that certain dish for the occasion.

Do you have a friend who's mom or family member makes cakes? Is there a local shop near you that does cakes and baked goods? Some of the local bakeries in my area offer such nice cakes, and they aren't expensive at all.

If you can't get family members to help out with the cooking, could you get some college friends to help you out?? They might like to spend the day at your place, helping with food and all of that. You can make that punch with juice, ginger ale or 7 up and sherbet. That's always a something nice to have at picnics and get togethers where you don't want to serve alcohol. You could also get some bottles of sparkling cider, which tastes really good, and looks just as pretty and fizzy as champagne.

Hope it all works out for you! It's so nice of you to put it together!