@Gardyloo Thank you, thank you, THANK YOU!
I had the most wonderful visit to the Museum of Flight. Got a taxi no problem after stepping off the ship. Cost was $28. The taxi driver helpfully pointed out that the museum campus is both sides of the road as he dropped me off, telling me to make sure I visit both sides.
I was there for over three hours. It was everything I'd hoped it would be. The "tower" with a view over the adjacent runway while listening to the Air Traffic Control chatter was an unexpected bonus.
I got the bus back into downtown Seattle no problem at all. They were every 15 minutes. It went through some - ahem - "interesting" parts of town on the way back, which gave me a flavour of Seattle I wouldn't have got from a cruise ship excursion.
Given the time when I was travelling back, I just stayed on the bus until near Pike Place Market and had a mosey around and a pint in a brew pub before walking to the ship.
Thanks again for the wonderful information. It was so, so much better than any organised excursion would possibly have been.