Parboil the onion. Cut the onion and tomatoes into quarters and cut the zucchini into 1-inch chunks. Brush the vegetables with the remaining olive oil, and place on skewers. Grill, turning often, until lightly charred. Chop the vegetables, toss them into the pasta salad add the basil and serve.
Many people fear picnics can take the chill out of their favorite slaw.
A friend once told me of a trick she used that served two purposes. After she making the slaw, she would freeze it until time to pack the picnic. The frozen container helps keep the other food cool, and the slaw is thawed by meal time. This works best with vinegar-dressed slaws. Add dressing to chopped slaw, tossing well to cover.