503 is a hoot, if you like the back highways. It skirts Mt. St. Helens. But that far south I5 isn't a "corridor", it's just a freeway. If you want a fun bypass, run I5 to Chehalis, turn left at Chehalis on Highway 6, hook up with 101, run to Port Townsend, take the ferry to Whidbey Island, cross Deception Pass to the mainland, and Robert's your mother's brother -- no Olympia, Tacoma, or Seattle.
If you want the less mileage route, go I5 - 12 - 7. Pick up 161 in Eatonville and drift to 410. Take a left on 169, hook up to 18N to bypass Seattle, and then cut off on the Fall City road, which is just about at the intersection of 18 and I90. At Fall City you pick up 203, which is the highway along the Snoqualmie that I mentioned in a previous post. 203 T-bones highway 2 at Monroe.
Both of these routes are scenic and fun to ride. Both go through the small towns and over some minor passes. The 101 route will have less traffic.