Here are 3 different routes from Seattle to Guemes: fast, semi-scenic, and super-scenic , along with points of interest to explore on the way -- if you have some extra time.
Semi-scenic has one extra ferry (besides Guemes), and super-scenic has two. The crossings will make it take significantly longer than the fast route, and the ferries can be very busy on Fridays or weekends. But it is a fun way to see some more interesting parts of the region, and we love our ferries here. Follow the posted signage and drive on.
Fast: You'll be on I-5 most of the way. This is what Google Maps will tell you to do. No shame! It gets prettier after you pass Everett. We added some suggestions on the map (blue pins) around where you'd turn off the highway, if you want to stop for a bite on the way.
Semi-scenic : From Seattle, Take I-5 to Mulkiteo and take the ferry to Clinton (20 min). Then drive north through Whidbey Island, stopping for lunch in Coupeville (try the mussels!). Deception Pass Bridge spans the other side so you don't need a boat to get off the island.
Super-scenic : This is a fun but long route, and the ferry gets very full on Fridays/weekends -- which might delay you significantly. We recommend this for returning to Seattle on Monday only:
From north to south: Drive south to Deception Pass and stop for a stretch and a selfie with the bridge. Carry on through Whidbey Island and admire the prarie as you drive to the Fort Casey ferry. Take the boat across the water to Port Townsend and enjoy lunch at one of the many restaurants in the old port's buildings, and get coffee to go from Better Living Through Coffee. Check out Finn River Cidery and/or Poulsbo ("Little Norway") on the way to the Bainbridge ferry terminal. Take the 30 minute crossing and admire the skyline view at sunset.