Ever thought to yourself, "Hey, I want to go to Seattle for a weekend, but I'm tired of riding the traditional STP route and don't want to pay for Amtrak fare?" Well, here's one for you!
This basically pieces together two of the brevets I did last year -- ORR's Olney Gothic Logger 300 and the last hundred or so miles of SIR's spring 400K.
You start in Portland and end in Bremerton. Assuming you want to get across the water to Seattle itself, you'll need to plan the ride so you can finish and hop on the ferry, which runs roughly once an hour between 6am and 11pm. Of course, this route can also be used in reverse, though I haven't yet written out a cue sheet for that way.
If you go north, you'll get the ride's only two semi-tough climbs out of the way in the first third of the ride. Highway 30 to Scappoose is, of course, totally flat, before heading over a lumpy Scappoose-Vernonia Highway that goes up to about 1,400 feet. There's also a great dirt path/trail/road which parallels this that I'd highly recommend as an alternative routing with the exact same distance.
From there, it's pretty familiar Oregon Randonneur territory -- out OR-47 then continue on OR-202, past the Birkenfeld store and towards Astoria. You do get back up to 1,200 feet or so when you go over the coast range, but it's a pretty relaxed climb, and on a backroads area with very little traffic.
After descending all the way down to just 70 feet and rolling through lovely Olney, OR, you continue into Astoria, where the rather terrible Astoria-Megler bridge awaits. It's long, nearly shoulderless, and usually windy. It's also well-signed for bicycles. Ride it, wait for it to be over, then enjoy continuing the rest of your ride on solid and wide roads.
Head north on WA-4, then meet back up with 101, which you'll take for a full fifty miles before turning inland at Montesano. Unfortunately you don't get many actual ocean coast miles, but it's still a nice semi-remote highway.
From Montesano, you roughly follow alongside -- but thankfully not on -- WA-12, on backroads through towns like Brady, Satsop, Elma, and McCleary. At mile 209 in Kamilche, you rejoin 101 (which is much busier here) briefly, heading for Shelton then WA-3 which takes you most of the way to the ferry.
After the Oregon Coast range, you never go back up over 500 feet of elevation. As is one of the perks of one-way rides, you can plan your trip and hope to get about 200 miles of tailwind riding (usually out of the south in winter/early spring, out of the north for summer/early fall).
Do keep in mind service options and availability for late-night riding -- there's a convenient 24-hour Safeway in Shelton at mile 216, but otherwise you might face long stretches without food available.