In most cases, going into SEA, the left side of the plane will see Mt Rainier, the right side will have a spectacular view of the North Cascades, including Glacier Peak and sometimes Mt. Baker. Leaving SEA, the left side of the plane will see Mr. Rainier and the right side will generally see Mt. St. Helens. I don't think you can go wrong with a window seat on either side, although if they haven't seen Mt. Rainier close up, the left side would be the choice.