We'll be arriving at SeaTac just before noon on a Sunday in August and driving to Anacortes for our first night. We don't plan to stop in Seattle.
After we get out of the metro-Seattle area, we'd prefer to get off I5 and drive through smaller towns. Considering Mt Vernon or LaConner or just seeing nice scenery on back roads, but still heading to Anacortes. Another option is to go directly to Anacortes and then visit/walk the parks. I believe it's Washington Park (?) on the ferry side of town.
We've had some great REAL milkshakes a bit further north/east of this area before. Any places known for shakes west of I5?

Don't know about the milkshakes, but LaConner is a nice town in which to break up your drive. We enjoy walking the busines district and "window shopping."

Guess I forgot to ask if anyone had more suggestions besides those that I put down. Are LaConner or Mt Vernon worth stopping to explore? If we stop at La Conner, are the roads public through the reservation to Rt 20?

I would stop in La Conner over Mt Vernon. The easiest route to go on to Rt. 20 west is to go back out to the rotary outside of town and north from there (it's clearly marked).

I don't know if they do shakes, but there's great ice cream at the big fruit/farm stand on the way from I-5 to La Conner (the name is escaping me at the moment but it's right on the road--left hand side, you can't miss it).

If you have time...just before you reach Anacortes, take hwy 20 (left) to Whidbey Island. It's a fairly short drive (20 minutes)and will take you over Deception Pass. It is the most beautiful area - quite like an enchanted forest. Just across the bridge, you will find Deception Pass State Park where you can camp, hike, go to the beach or splash around in Cranberry Lake. Another 20 miles up the Island and you'll find Coupeville which is a quaint little harbor town. Near the ferry there, you will also find an old, well-kept military fort called Fort Casey that is worth exploring and having a picnic. If you find yourself out that far, hop the ferry (30 minute ride) to Port Townsend which is another fantastic little harbor town. You can stroll around and eat at Finn's (moderately priced seafood on the waterfront) or go up the street to Tyler's which a fab little bakery. Have fun!

If you get to La Conner, go to Kirstin's restaurant...it's the best!! small, good appetizers, lovely food, and not too pricy. Also, lots of cotton shops and natural fiber stuff, or can sit out on the water side and watch the boats come and go. We have a beach house up that way and never tire of it.

I think we will opt for La Conner; I've heard lots of good things about it, but just haven't stopped there the times we've been in the area.
Thanks for the suggestion of Whibey Island, Kiowas. We spent 3 days near Coupeville two years ago, and had a great time. We'll be arriving from the east coast that Sunday, so don't want to do too much extra driving.

Just a little forewarning - you don't mention the dates you are traveling - however, there will be major traffic issues on I-5 Northbound in Seattle around the I-90 intersection. Multiple lanes will be closed from August 10 - 30th. This will certainly cause a delays and frustrations for anyone traveling from SEA-TAC Northward. Just an FYI.