An idea that falls into the "why didn't we think of that" category: picnic baskets lined with insulating Styrofoam and rattan baskets with galvanized liners to keep food and drinks cold all afternoon:

Above: The Rattan Party Bucket with galvanized lining; prices start at $40 for the small at C.S. Post & Co.

Above: The Peterboro Tailgating Cooler Basket is $44.

Above: The Peterboro Ice Master has a removable Styrofoam insert to keep food cool; it's $49.