Don't bother going south of Burlington on I-5. Take SR-20 west towards Anacortes and then south over the Deception Pass bridge onto Whidbey Island, then to the Port Townsend ferry. Interestingly, the ferry ride is considered part of SR-20. Continue south from PT to the junction with US-101. Head west to get to the WA coast or east to go down the west side of Hood Canal.
US-101 down the Pacific coast is a fairly decent road, two-lanes most of the way. Apart from Ocean Shores, most of the towns are small and facilities are a bit limited. It does have good access to the Olympic NP and the wettest place in the US!
Hope you have an enjoyable trip. If our current weather holds, it will be quite spectacular.