OK here's what I'm planning to do on my next picnic:
Take a large loaf of bread, preferably something crusty and 'rustic' and turn it over. Cut out a large circle, leaving about 2cm border and remove this disk of crust. Then pull out the soft bread, leaving a bowl shape in the loaf. Fill the bowl with anything that you like eg. mozzarella, salad, cold chicken pieces, ham, hard boiled eggs etc. Replace the disk of crust, wrap the loaf and store in the fridge until required. If it's a large loaf, it may be best to weight it down slightly to compact the contents.
Take a sharp knife with you and when you get to your picnic site, just cut the loaf into wedges and eat. Nom nom.