The Doubletree has got park 'n fly rates for stays of 7, 10 and IIRC 14 days so if you have an early AM departure or arrive back into SEA late, it's an option. You can do your stay at the beginning or at the end. The DT shuttle is every 15 mins 24/7 and it's even walkable from the airport in a pinch.

I remember seatacpark starting at $0.99 when it first started. Still have the 5 free days from a punch card completed 2 years ago. Thankfully these don't expire.