Picnics are essentially childish, so if you have children (or can borrow some) let them choose the food, even if this means sponge cake with green icing rather than bruschetta and Chardonnay. Even better, ask them to suggest a recipe culled from a children's novel.
I remember being constantly tantalised by exotic dishes described in mouth-watering detail in books. What Katy Did begins with a picnic, conveniently prepared by the family cook: baked meat pies, cakes, cookies and ginger pop.
American food of the traditional chowder and clam-bake variety was particularly intriguing for a British child back in the Sixties. Remember the raspberry cordial incident in Anne of Green Gables? The delicious idea of waffles and maple syrup would keep me awake at night, while Laura Ingalls Wilder's Little House books were packed with pioneer-style meals that provoked a sort of appalled fascination: hominy grits, sour-dough bread, salt-pork, cornbread, maple candy ("candy" sounded so much better than sweets) and, best of all, cornmeal mush with chicken gravy. Attempts to persuade my mother to cook cornmeal mush fell on deaf ears.
While American food still has the power to fascinate, today's young citizens of the European Union are probably too familiar with pasta and croissants (in my childhood they were still called "French crescents") for European food to have much outlandish appeal. Rifling through Victorian children's books or ancient cookery texts might yield more interesting results: Simnel cake, maids of honour and lardy cake would travel well, and you could take Peter Rabbit's camomile tea in a flask.
Traditional fairy tales offer food with sinister, or even poisonous, aftertastes: Snow White's apples and the Wicked Witch's gingerbread house studded with sweets (gingerbread shapes, if not houses, are easy to make). But the fantasy food that translates most easily into kitchen reality comes from comics. It only takes a little ingenuity to stick a couple of cardboard cow-horns into a pie fit for Desperate Dan.