The first thing that comes to mind is Nasturtium. You can't find Nasturtium in cell packs at the garden center because they don't transplant well, but they grow easily and quickly from seed and thrive in amazingly hot, dry, and unfriendly sites. They'll tumble over the sides of the window box and the neat round foliage contrasts beautifully with Asparagus Fern. (A. sprengeri). Other annuals for sunny containers include Dragonwing Begonias, Celosia, Dahlberg Daisy, Dusty Miller, Gerbera Daisy, Melampodium, Petunia, Moss Rose (Portulaca), Salvia, and Vinca. If you grow Geraniums or Marigolds, they've got to be deadheaded. (There are some Marigolds that bloom clean yellow so they don't clash with other annuals.) New Guinea Impatiens are colored vibrantly and thrive in full sun, but they'll wilt quickly if the window boxes dry out. There is a whole line of sun Coleus for bold foliage. As you shop the garden center, mix and match annuals in your cart until you find a combination you like. When you plant, leave spots to poke Nasturtium seeds down into the soil.