Sometimes simpler is better. Skip the souped-up coffee drinks and order a small, zero-calorie green tea at Dunkin' Donuts. You’ll get a cupful of flavonoids, antioxidants that have been shown to prevent cancer and improve heart health. You’ll get a mental boost, too. Green tea contains theanine, a compound that enhances energy and concentration, without the jitters you get from all the caffeine in a cup of coffee.