In a medium saucepan, bring the water, 2 tablespoons olive oil, ¼ teaspoon salt and ⅛ teaspoon of pepper to a boil over high heat. Add the couscous and turn off the heat. Cover the saucepan with a lid for 10 minutes. Fluff with a fork and allow 10 minutes to slightly cool.
While the couscous is cooking, make the vinaigrette. In a small bowl whisk the lemon juice, the remaining ¼ cup of olive oil, and the remaining ⅛ teaspoon each of salt and pepper.
In a large bowl, toss together the grapes, olives, and cucumbers. Add the couscous and toss to combine. Pour the vinaigrette over the couscous and gently toss to combine. Sprinkle with the feta cheese and mint. Serve at room temperature or cover and refrigerate for up to 3 days.