For fish I often get cod - it isn't heavy, and you can get it ready very quickly.
Salad dressing is perfect. Pour some on, and bake it twenty minutes or so at 375 and that's it. If I have a strongly flavored dressing that's enough, but the other day I had Italian, so first I spread mustard (Gulden's Spicy Brown, if you must know - totally forgot I had some dijon in the fridge) overall, then some Italian seasoning, and then plenty of dressing. Just right.