Plain, non-fat yogurt instead of mayonnaise. Now I have never tried this baking so I'm not sure how well it'd work but one of our favorite uses for it is in chicken salad sandwiches. I love chicken salad sandwiches but would feel guilty for all the mayonnaise that's in it. I started doing half mayonnaise, half yogurt and then switched over completely to yogurt and I can't even tell a difference!
Simple but a tad bit healthier!