Peel sweet potatoes, and cut into 1-inch cubes. Cook in boiling water to cover 20 minutes or until tender; drain.
Stir together mayonnaise and next 4 ingredients in a bowl; chill. Gently toss sweet potato, banana, apple, and grapes. Cover and chill 2 hours. Serve sweet potato mixture on lettuce leaves, if desired, and serve with mayonnaise mixture. Garnish, if desired.
* 1 (16-ounce) can sweet potato pieces, drained and cubed, may be substituted.