Drain pineapple, reserving 1 tablespoon of the juice. If using strawberries, cut them in halves. For the fruit mixture, in a medium bowl, combine the pineapple, strawberries or grapes, apple, and orange sections. Toss lightly to mix. For dressing, combine yogurt, poppy seed, and the reserved pineapple juice. To serve, line salad plates with lettuce leaves. Arrange fruit mixture on lettuce. Drizzle dressing over fruit.