Flyting to SEA, make sure to get an "A" side seat. Opposite for the way back obviously. The reason being if its clear you get a spectacular close up view of Mt. Rainier and the rest of the Cascade range. Actually the other side of the plane isn't half bad either as you can often see Mt. Baker and others peaks.
Often times the pilot will "dip" the wings to give people a better view.
I've done this flight so many times and it never gets old.