If you're going to hire a caterer, choose one that uses seasonal, local and organic ingredients in their dishes. It not only means you'll get delicious food that will stand out both in appearance and taste, but you'll also support local growers whose foods have traveled far shorter distances from farm to plate than foods grown on large, mass-produced farms. Local and organic produce is also more likely to be grown naturally, which means it typically requires less -- or no -- pesticides or excessive watering that can also take a toll on the environment.
During your planning, consider your leftovers, too. Look for a caterer that participates in a composting program or one that donates any good leftover products to a food bank or shelter. And if the caterer doesn't take the extra steps, go ahead and make them yourself. There's probably a community or school garden that would be grateful to get some extra goods for their compost bins.