How can I ensure a successful picnic?
I'll leave advice on guest lists and play lists to other experts, but from a food standpoint, there are two primary measures of picnic success: Everyone enjoys the food, and nobody gets sick from it.
On the safety front
1. Keep cold food cold. Cold foods should be stored at 40 degrees or less until serving. Using an insulated carrier and frozen cold-packs, this shouldn't be a problem. Don't let perishable foods sit out for longer than two hours or, if the temperature is higher than 90 degrees, one hour.
2. Separate the raw from the cooked. If you are intending to grill, pack raw meat in sealable bowls or bags so that it doesn't come into contact with anything else. After cooking, make sure you don't place the meat on any surface or platter that contained raw meat.
3. Don't blame the mayonnaise. Because of its acidic nature, commercial mayonnaise actually provides a relatively inhospitable environment for bacterial growth. Canned tuna is more likely to "go bad" if it hasn't been dressed with mayonnaise. That said, mayonnaise will not protect tuna, potatoes, eggs or macaroni indefinitely, so make sure you adhere to rule No. 1.
On the enjoyment front
1. Make a list. Write down everything to take including, but not limited to, napkins, flatware, plates, cups, a corkscrew, salt, pepper, wipes and insect repellent. Also write down all the food you intend to take so you don't leave anything in the refrigerator.
2. Portion before packing. The less cutting and spooning you have to do at the picnic site, the better. So try to serve food that is already in serving pieces such as cut-up chicken rather than a whole bird, brownies instead of cake.
3. Be vigilant about sogginess. Dressing will destroy a green salad that is mixed ahead of time; lettuce and tomato will compromise sandwiches. Try to keep wet / moist things away from dry things until the last minute.