Personally, I'd stay in one of the airport hotels (make sure it has 24 hour shuttle service to the terminal). Make your reservation and, on your way back from Mt. St. Helens, turn your car in, call the hotel and get its shuttle to the hotel. A 5:30 a.m. flight means getting to the terminal around 3:30 a.m. That sounds absolutely awful, I know, but the security lines can jam up with people getting their early flights to the east coast. Not having to turn your car in then will be one less thing to do...and at that time of the morning anything that delays getting through security and getting a mug of strong coffee should be avoided.
I agree with oregonpoppa 100%.
That sounds like perfect advice. Thanks!
Agree with this advice of staying near the airport.
Not sure on pricing for your stay. We stayed at the Residence Inn at Cascade Station. We used both the hotel shuttle and the light rail. The light rail station is across the parking lot for the hotel and is the last stop before the airport.
Enjoy Mt. St. Helens. Visiting there shows both the devastating power and amazing rejuvenation powers of nature.