Interstate 90 over Snoqualmie Pass in the state of Washington is unique in the Interstate Highway system in the US, in that it is a designated National Scenic Byway. The Mountains To Sound Greenway Trust (http://www.mtsgreenway.org) has acted to protect the route of I-90 from Seattle to Thorp, WA, which is the portion of I-90 through the Cascade Mountain Range. Many scenic and unique places in the Cascades can be quickly and easily accessed from the Seattle area via the Mountains to Sound Greenway and I-90. For example, Snoqualmie Pass is only a one hour drive from downtown Seattle (in good weather).
(http://www.seanet.com/~pfitech/I-90GeoTour.pdf) <-- Here's a "Geo-Tour" taking you from Seattle to Vantage, WA, a driving and hiking tour guide in PDF format that you can print and take with you, showing you many of the geological and historical points of interest along the way, as well as tips on nearby short side trips and hikes!