So once you’ve seen Seattle from above at the Space Needle, the next thing to do is see Seattle from below. The Seattle Underground.
Did you know that in 1889 Seattle burned down? And the city couldn’t handle it’s shit, and I mean that literally, the sewer was above ground and Seattle was below sea level, so the shit was coming back in with the tide. So when they rebuilt, they rebuilt on top of the old city and there is a maze of underground sidewalks and store fronts.
The Underground Tour is a punny funny tour. I highly recommend it, especially if you’re a history buff. It starts from the Pioneer Building in Pioneer Square. I might recommend purchasing tickets in advance during the busy season and on weekends, but otherwise, you can probably walk right in and grab an upcoming tour, during the busy season they are offered every half hour. It is a walking tour with several sets of stairs, but it isn’t strenuous or hard. 
After the tour you can wander around Pioneer Square a bit more and check out the old buildings, architecture, and shops:
I recommend an early morning weekend Underground Tour. Because after the tour, you can then hop on a water taxi over to Alki Beach and be gluttonous at Salty’s Brunch. A huge brunch buffet with waffles, pancakes, french toast, crepes, omelets, bacon, potatoes, mac and cheese, seafood, and a ton of desserts. Plus, a stunning view of Seattle and the water.
It’s a little pricey, but it was the best waffle I’ve ever had. Ok, waffles. I went back for seconds. Maybe thirds.
Planning a trip to Seattle? Check out my other Seattle guides: