Have to disagree with the two previous posters. I thought the STP was a great route and would do it again. Most was rural with regular friendly small towns.
I never said I didn't like the STP routing. (Well, I'm not particularly fond of the urban stretch from Spanaway to Seattle.) I just said it wasn't thrilling, compared to say, the Oregon Coast or Columbia Gorge, two other rides in the vicinity. For me, riding the direct route between Portland and Seattle is usually more a matter of getting from point A to point B than the ride itself. But to each their own.
To mix it up a bit, one could take the ACA Pacific Coast Route from Castle Rock to Centralia. It stays east of I-5, whereas the STP stays west. It's pretty similar scenery, but has the advantage of going right past Lewis and Clark State Park, which has a hiker/biker site. Watch out for the dogs, though.
If I were to do it again any time soon, I would definitely try to integrate Vashon Island into the route like waynemyer suggests, or going for a easterly path along the Cascades like bogiesan proposes.