Don't slice the zucchini too thin before you grill them since they will shrink down. We like it between 1/4"-1/2" thick before grilling. That way you'll get a nice lit bit of crust balanced against the softer interior.
In the same dressing bowl, add the zucchini, quinoa, cherry tomatoes, red onion, feta cheese, beans or chickpeas, and basil or corn. Gently toss everything together.