You could start a mesclun mix, it only takes 3-4 weeks to harvest, and it's a good choice for a window box. Just "mow" with scissors, and the greens will regrow until the first frost.
You can also start onions now (for next Spring), kale (I love dinosaur kale), spinach, or satsoi. In about 6 weeks, you can plant some garlic (for next summer.)
Subject: Re: [roofdeck] Something Fun to Grow (and eat!)
From: [address removed]
To: [address removed]
Date: Wed, 14 Aug[masked]:27:12 -0400