Portland to Seattle
The difference between the route we took on that trip and this one, though, is that we took Highway 30 between Portland and Longview. The traffic on that stretch of road drove Carolyn to distraction, which was the impetus for me to scout out the alternate route on the Washington side. It's a more challenging road, but more scenic and more pleasant, too, so on the whole, probably a better way to go. If you're interested in making the trip to Seattle, you can choose either way to get to Longview -- just know what the pros and cons are of each.
From Longview, the route takes the tried-and-true StP route all the way to Spanaway, with a modification between Napavine and Centralia -- my route stays out of Chehalis, instead going on 603 to Shorey Road, joining up with the main route again near the Centralia airport.
In Spanaway, the route goes around the back of Spanaway Lake on the Spanaway Loop Road, and then takes a combination of Ainsworth, Sheridan, and M Streets to get into downtown Tacoma. In Tacoma, it's best to try to stay on top of the ridge above the urban core, because you'll just have to climb back up it if you go down. From there, wind through the neighborhoods in the northern part of the city to Point Defiance, where you can take a ferry to Vashon Island.
This is the inspired part of this route: you don't have to deal with the wearisome Puyallup-Sumner-Auburn-Kent-Renton stretch of southern Seattle suburbs. Instead, once you get through Tacoma, you take a nice ferry ride, pedal across bucolic Vashon Island, and then take the passenger ferry from the northern tip of the island right into the heart of downtown Seattle. Much better. Be sure to stop in at the bakery in the village in the middle of the island (should be at around 176th street.)
Here is a link to the detailed map on the gmaps pedometer.