If you want to make it really easy you can take US 12 from Lolo, MT to Morton, WA then Highway 7 (aka Mountain Highway) to Eatonville then Highway 161 (aka Meridian) into Tacoma and then a ferry to Vashon Island then ride to the ferry on the other side of Vashon and take the ferry to West Seattle and bike to Renton from there.
US 12 runs over Lolo Pass in Idaho/Montana and White Pass in Washington. It connects Aberdeen, WA to Detroit and can get repetitive being on the same road for so long.
I followed it a lot on my tour last summer. Was glad to see the end of it in Michigan. Before I-90 and I-94 it was the main thoroughfare but has obviously been supplanted by the interstates.