If you cross the Cascades on I-90 into Seattle, the Tacoma Narrows Bridge on WA-16 is your best bet. The limited access highway through the Kitsap Peninsula will be minimum delays, though traffic will be moderately busy.
I-5 from a bit north of Tacoma down past the turnoff to WA-16 is the worst construction/ traffic currently in the state. And is going to be for years as the rebuilt of the section through Fort Lewis/McCord AFB gets started.
Personally I'd prefer coming across the Cascades on US-12 and up I-5 to Olympia to US-101.
An alternate of US-2 and the ferry might have the easiest traffic, and might not. It will certainly be the most expensive. Before getting to the ferry terminal, unhook the toad and have the wife drive it separately. Cheaper than towing it which adds to total vehicle length.
Also, do not take US-2 westbound on a Sunday. A year ago people were seeing 8 to 12 hour drives from Leavenworth to Monroe on US-2 eastbound on Sunday afternoons.
