There are many "eat outside" holidays. Eating pie outdoors (especially in the summer) brings its own special challenges. You'll need a pie that will travel well and hold up to the summer heat. Custards are out. Along with anything that contains eggs or dairy. And you probably don't want to make anything that requires lengthy oven time. If you opt for tart, transport it in the tart pan so the sides don't collapse.
Try:
Or if you need a main course instead of dessert, a BBQ Pie is great because there's no dairy to rot in the sun.