We are doing the same thing, late flights after our cruise. We will sign up for Port Valet, so we will have no luggage and will already be checked into our flights.
We are flying in 3 days early, so will have already seen a fair amount of downtown Seattle.
So we decided to rent a car from the airport for just the day. I have been doing some research on what to do outside of the city. We crossed off Mt Rainier, as we will have 3 kids with us and want something more active than just sightseeing. My plan so far is to go to Snoqualmie Falls (walk the trail) and to Fall City Wallaby Ranch (we already have reservations). I think we will all enjoy both stops. But it will not take up the whole day. We plan to be back at the airport early, but our flights don't leave till 10:30ish, so don't want to sit at the airport longer than we have to. Any suggestions for the rest of our day in that area?