Lived there for a decade.
The boardwalk, the Aquarium, the Sound ferry tour, the Experience Music Project (EMP), the Space needle, Chittenden locks, Alki beach (If you are a road biker, you can ride from Kent to Alki Beach on the green river trail ~25 miles, and enjoy muffins from the Alki bakery, a decent coffee/Americano from Tulleys, and hang out at the beach.
Pike's place market is a great visit for the casual tourist, and it includes the original Starbucks, plenty of fish, flower and craft merchants, and if you find the right corner, there is/was a fresh mini-donut stand that makes/made hot, fresh, non-diet-friendly mini donuts; plain, powdered sugar or cinnamon sugar.
If you hold over on a weekend, you can ride up to Snoqualmie pass and hike to Franklin Falls, or to the natural Denny Creek Slide. Or if you are really ambitious, a 2000 ft. vertical hike to Lake Melakwa. (4700 ft. elevation). Tiger Mountain is also nearby and open to the public for hiking and off-road biking. For the road biker, the Burke-Gilman trail is outstanding, and it leads to the Woodinville brewery that makes Mac-n-Jacks African Amber Ale.
If you're bringing kids, the Boeing air museum is fantastic as is the Chiluly glass museum and the Pacific Science center.
In spring, up North, the Skagit tulip festival is a must-see.
If you have a valid passport, Vancouver and Victoria BC offer a plethora of experiences, such as the Capilano Suspension bridge.
South of Seattle - Salty's at Redondo beach is a favorite restaurant. Driving out to the Kitsap penninsula or taking a ferry is a good way to see more of the Puget Sound and maybe see a submarine or two.
The Puyallup (WA State Fair) typically runs in September, with plenty of music, attractions, carnival food, etc...
You might want to bring your REI/Columbia/NF rainjacket, socks and sandals, throw in a Starbucks and you'll blend in with the locals.
And if you're there on a rainy weekday, driving from Kent or Renton to Bellevue on 405, you might want to leave a bit early.....
Best wishes for a great time in the PNW.