After living in Seattle many years I decided this trip is the perfect way to show guests the best of Puget Sound. I never tired of it myself.
Start by taking I-5 north from Seattle to Everett, then west to the ferry landing at Mukeltio. Once on Whidbey Island (the largest island in the Lower 48 due to a Supreme Court ruling that Long Island, NY, is not a true island), turn right soon after the ferry landing to Langley, a 19th century charmer on a bluff overlooking the sound and mountains. Some good shops and also good places to eat.
From Langley, go north to Coupeville, another 19th century fishing town on a bluff overlooking Penn Cove. Several shops here also, plus a local museum.
Follow the road along the coast on to Captain Whidbey Inn, also on Penn Cove. This is a good place to stay, and also to have a snack or meal. The owner of the inn owns a large sailboat he charters for cruises.
After you 'do' Captain Whidbey, continue on north through Oak Harbor (not much of interest to tourists here) to the northern end of the island at Deception Pass State Park. This is one of the most dramatic places on Puget Sound because the water rushes through a narrow gap at each tide change, creating standing waves and whirlpools. It is the most popular park in the state, but so large that it seldom feels crowded.
From here you have the choices of seeing Anacortes, a nice town that is the jumping-off place for the San Juan Islands, or turning back south to LaConner, a former fishing town given over almost entirely to tourism.
Continuing on the perfect-day route, go east on the main highway until you find the signs to Bow and Edison, which will take you along the sound to Chuckanut Drive. Then follow it north. It was the state's first scenic highway is remains one of the prettiest highways in the state. It follows the edge of the mountains that drop into the sound, and you have your choice of three or four good restaurants, some described elsewhere here.
Churkanut Drive ends in the Fairhaven District of Bellingham. If you plan to return to Seattle, I-5 will get you there (85 miles) in about an hour and a half.