A mix of some good info in this thread, and some really bad misinformation/ignorance.

Overall good: yes, Kens should still be a going concern, and an obvious place to drop your bag. Give them a call first to check open hours and what you can expect to pay. But consider checking that monster for your next leg - do you really want/need to lug that along with you? Might be cheaper to check it than store it for half a day...

The"airplane museum" is the Museum of Flight (MOF). It's located on Boeing Field, which is NOT on the other (far) end of the Seattle area. It does NOT require a reservation. Boeing Field is easy and quick to get to from Seatac if you have a car. If you don't, it's a bit of PITA to reach. Without a car, I'd take a taxi (without bad traffic, about 15 minutes dive from Seatac). The MOF is great if you have an interest in aviation, probably a yawn-fest if you have none. BTW, the other aviation museum/facilities that were probably referenced are at Everett's Paine Field, much further away to the north. There are actually a bunch of interesting facilities there, all of which seem to have been conflated (all of which have more restrictive hours and availability): there's the Boeing Factory Tour, Flying Heritage Collection, Historic Flight Foundation, etc. Those are not really practical for a quick visit unless you are really dedicated and organized, have planned ahead, and have wheels.

Note that traffic in Seattle is generally nightmarish now (the city - and the airport - is so overcrowded I hardly recognize it anymore, sigh...). Allow yourself way more time than you think it could possible take to get anywhere. It's Gay Pride weekend, too, (plus multiple sports events going on) which will make traffic worse than usual. At least Obama's just left town (an hour ago), so large swaths of the city won't be completely shut-down iike they have been for the past 24 hours, just the regular gridlock as is the new norm.