You could plant any of the cool-weather greens choosing them according to your timing and the size of your pots. For fast growing, shallow rooted greens you could try arugua or broadcast a mix of lettuces for cut-and-come again culture. They don't like hot weather so plant in fall or try starting them in shade in August. Or if you have more time and some deep pots, plant rainbow chard -- plant them thickly and thin, selecting the brightest colored seedlings. They are very pretty and chard appears to be a little more tolerant of summer heat than other greens.