Though I lived in Tacoma for six years, I never spent a lot of time in Seattle. Being car-less, busy with school, and spending the majority of my freeway time driving south to Eugene with Ben, there weren't a lot of opportunities to explore the big, beautiful city just thirty minutes north. I'm surprised I didn't go to more in Seattle in those six years, but luckily it's only 3 hours (on a good day) from Portland!
I've spent a lot more time on Bainbridge Island, a tiny island in the middle of Puget Sound that looks across at Seattle and is accessible primarily by ferry boat. My grandmother moved to Bainbridge when I was about 4 years old and has been there ever since. Most family trips as a kid had Bainbridge as a destination and I really fell in love with ferry rides and the beautiful views of the mountain ranges (Olympics and Cascade), the glittering water and the Seattle skyline.
I highly recommend taking a trip across the Sound if you're ever in Seattle. The boat ride is iconic (Grey's Anatomy, anyone), takes about 35 minutes, is probably cheaper than riding the ferris wheel on the waterfront, and drops you into a charming, walk-able downtown area.
My other Seattle recommendations are to find someone that lives there and get carted around by them! Locals are always better resources than rolling the Yelp dice or trying to find a comprehensive city guide. On this particular trip, our best friends took us into Ballard. We took a peek at the Ballard Locks on Lake Union, had brunch at a very popular spot called Portage Bay (highly recommend and with reservations), wandered around and window-shopped, and then tried all of the Ballard-brewed beers at Ballard Brewing Company. It was a gorgeous day and the streets were filled with people enjoying the weather and the city!