There are two buses that go between downtown Seattle and Seatac airport—194 (aka the fast one) and 174 (aka the slow one). The 194 doesn’t run on Sunday, so when I fly back home on Sunday (which I seem to do often) I get stuck on the slow one.
I get a nice leisurely tour of the dive bars, motels, and hole in the wall ethnic restaurants on 99 S of Seattle though.