The San Juan Islands archipelago is a group of islands north of Seattle. To get there, drive to Anacortes, about 75 miles north of Seattle (1.5-hour drive). Then take the ferry to Friday Harbor on San Juan Islands. There are more islands, including Orcas Island, that are popular. San Juan Island is the busiest, I think, and has the cute little town of Friday Harbor. In Friday Harbor, walk around, eat a delicious seafood lunch and take a whale-watching cruise. Seeing killer whales (orcas) out in the water is amazing.
It's a great day trip, or you stay on the islands for a little getaway.