Prepare quinoa with the 2 cups water according to package directions; cool.
In a large bowl, combine prepared quinoa, Grape-Nuts cereal and grapes.
In a small bowl, whisk together oil, lemon juice, honey, oregano, mint, salt and pepper. Add to quinoa mixture and gently toss to coat. Cover and refrigerate for at least 30 minutes, until chilled, or for up to 2 hours.
When ready to serve, add arugula, feta cheese, and walnuts. Gently toss to combine.