Hey Ben,
I am only 17 so I couldn't drive over there. My mate is 18, so our options were limited. We found one place and that was E-Z Rent-A-Car. It cost about $430 for the 2 days
But because the hotel was quite far from Boeing Field and Sea-Tac, it probably would've been a similar price getting around by taxi. Having a car was just easier and more convenient.
Cheers,
Brandon