Walk onto the ferry at the Downtown terminal station and take the 30ish minute ride to Bainbridge Island. You can walk into town (about 10 min walk) for lunch or wine tasting. If it's a clear day, it's a great way to get on the water and see Rainier, the Olympics, and Mt Baker.
One of my favorite tourist attractions! Short but cool and unique. Consider taking the monorail if you're coming from downtown (it was built solely for the 1962 World's Fair).
If you're looking for that nature experience but don't want to go far, I would recommend checking out the trails at Discovery Park. Best views are at West Point Lighthouse (but fair warning, there's a big hill to climb on the way back home)!
Historical building with a cool bar on the top floor and an outdoor observation deck. May need reservations.
I recommend Ray's Cafe which is the upstairs restaurant to Ray's Boathouse. Atmosphere is casual and sunsets over the Olympic Mountains cant be beat! Walk-ins only.
Agua Verde Cafe rents kayaks and SUPs on Lake Union. It's a great way to see the city skyline, houseboats, and Gas Works Park. Celebrate your hard work afterwards with chips/guac and a margarita.
Rent bikes at Recycled Cycles and take a tour up the Burke-Gilman trail (20 mile trail). Stop at Magnuson Park, Magnuson Cafe and Brewery or 192 Brewing Company.