Picnic food has to survive carrying, limited utensils, and whatever temperature the day provides. Choose dishes you can keep safely cool and serve without a complicated setup.
Pack perishable foods with suitable ice packs in an insulated container, and keep them out of direct sun. Bring only the amount you can manage safely. Shelf-stable unopened items can reduce the cooling burden when they suit the meal.
For texture, keep dressings separate and cut food into manageable portions before leaving. A large platter that needs a sharp knife and a level table may be less useful than a few individual containers.
Take clean utensils and a way to handle rubbish and leftovers. The pleasant part is eating outdoors; the meal does not need an elaborate arrangement that makes packing up harder than cooking it.
Further reading: FDA: storing food safely.