You can see ants and avoid them, but it's not possible to see, taste or smell dangerous bacteria that can cause illness if food is mishandled. With great weather and holidays, picnics are in season. However, it's also the season when more people become ill from foodborne bacteria. So, food transported without an ice source or left out in the sun at a picnic won't stay safe for long. That way, you won't have to worry about the storage or safety of leftovers.
Clean preparation is essential.
Foods to be cooked ahead should be cooked foods in plenty of time to thoroughly chill them in the refrigerator. At the picnic, keep the cooler in the shade. Keep the lid closed and avoid repeated openings. Replenish the ice if it melts. Use a separate cooler for drinks so the one containing perishable food won't be constantly opened and closed.