It's honestly more convenient and cheaper for me to just fly to Seattle and drive the 5 hours to Silverwood. Yes it's a long drive, but it doesn't end up taking much longer than tacking on a layover or two.
I found some cheap flights, but I think I need Washington to get to Phase 3 of their reopening plan before I could rent a car for leisure travel.
The last sentence also might rule out Spokane, unless they're ahead of the Seattle area. IMHO, it would probably be easiest (even though it would include a very long drive and/or multiple connections) to fly into Boise (will need at least one connection and is still a 7-8 hour drive), Salt Lake City (Lagoon will likely be an early-open park so one could also go there if flying in or out of SLC, even though it's a 10+ hour drive according to Google Maps), or Missoula (would be the shortest drive (about three hours), but would likely require two connections or one very long connection from most major cities).