Mountain to sound greenway sign.
Sign depicting Mountains To Sound Greenway, an umbrella planning entity along the I-90 corridor between Seattle and Vantage, WA.

By Robert.

The I-90 corridor, offers several nearby bike routes and many back roads.   It could be wonderful, but the I-90 bridge across Columbia River is not very good for bikes.  That is the weak link.  One can go quite a ways out of the way and bypass it.  Either going by way of Wenatchee (to the north) or Yakima (to the south).

Wenatchee has some bike paths across the Columbia. 

If one goes south from Ellensburg to Yakima, one can head east again and cross Columbia River at Vernita.  That's a better bridge than I-90 at Vantage, but I-90 can be done.  It's just a bit spooky.  It is legal at least.

Here are some bikeways along the I-90 corridor.

I-90 Tunnel and Lake Washington Floating Bridge Bike Path out of Seattle.

I-90 path ends in Issaquah.  One can use wide freeway shoulder until one can exit for Old Highway 10 business route into North Bend. 

Iron Horse Bike Trail starts up a hill south east of North Bend.  One can get there by road, or Snoqualmie Bike Trail.  Iron Horse starts at Rattle Snake Lake.

Iron Horse Trail.   Crosses Cascade Mountains.

East side of Snoqualmie Tunnel Iron Horse Trail becomes rough.  They are working on a trail all the way across the state, but only portions are currently finished.  I-90 shoulder is wide or one can remain on rougher sections of trail around Lake Easton State Park.  Then there are some other back roads and a trail from Roslyn to Cle Ellum.

Hwy. 903 Goes from Cle Ellum to Hwy. 970 which can take one to 97 (the Wenatchee route).

97 goes over Blewett Pass to Wenatchee where there is a better bridge across Columbia.  Hwy. 2 can take one to Spokane from Wenatchee.

Or.

Staying along I-90, one can head into Ellensburg on Old Hwy. 10 and then follow Old Vantage Hwy. all the way to the Columbia River.

Cross Columbia on I-90 bridge if you dare.  It is legal, but shoulder gets narrow on steel portion of bridge.  It is a narrow concrete ledge.  Watch out for wind.  If one is west bound, Ellensburg and Vantage area winds can be terrible.  Bridge can also have bad crosswind. 

Follow I-90 shoulder, or frontage roads when available, to Moses Lake.

Head north east through downtown Moses Lake and then find Wheeler Road which turns into Road # 3 and eventually Rosenoff Road.  Head east to Ritzville.

Then follow Danekas road, or I-90 on east. 

Follow I-90, or frontage roads to Spokane area where I-90 eventually becomes illegal for bikes.  Follow alternate routes (most likely Highway 2 into Downtown Spokane and River Front Park.  Then get onto the Spokane River Centennial Trail and follow it well into Idaho all the way to Lake Coeur d'Alene.

Heading on east from Coeur d'Alene, one can use I-90 shoulder over Fourth Of July Pass.  Then take Canyon Road, instead of I-90 to Cataldo where one can pick up a great bike trail all the way to Mullan, Idaho.  Trail Of The Coeur d'Alenes.

A short ways east of Mullan, one can get on Saint Paul Trail (like a logging road) up to Lookout Pass Ski area on Montana border.