US 12: Lolo Pass, across the Idaho Panhandle, is generally considered one of the best motorcycle rides in the NW US.
Two good options for getting to western side of Washington State... US 20 (North Cascades Highway) and US 12 (White Pass).
North Cascades is a great ride, but does get closed to through traffic, each year, late in the Fall/early Winter due to snowfall. Typically doesn't re-open until Spring (was even later opening, this year).
White Pass takes you right along the south side of Mount Rainier (dormant volcano, and one of the highest peaks in the lower 48), and isn't all THAT far from Mount Saint Helens National Volcanic Monument.
You can take US 12 out to the ocean (there's a section that is co-located with Interstate 5), and then head down the coast to Astoria, OR -- crossing the mouth of the Columbia River on a multi-mile set of bridge spans. From Astoria, you can either continue down US 101 all of the way through Oregon -- and into the California Redwood (sequoia sempervirens - the tall coastal redwoods) groves (Redwood National Park)... and down to San Francisco,
or, you can head back towards Portland, OR on US 30, and head south on Interstate 5.
US 101 down the Oregon Coast is a beautiful ride, but it might not be a great ride two-up. The road is in 'not particularly great shape' in places -- due to erosion of the hillsides above the ocean, and it is a long slog through the coastal towns. One option would be to take I-5 through Oregon, and cut across to the coast -- via US 199 -- between Grants Pass, OR and Crescent City, CA.