Having spent almost a week in Seattle again there’s a lot of stuff to do and see here.
The classics are quite quickly done, riding the monorail and visiting the space needle. Both were constructed for the World Fair in 1962. The needle has a restaurant at the top that spins while you’re eating to let you see the whole 360 degree view. Be sure to book!
Once you’ve seen the faboulous view from the needle, don’t miss the Experience Music Project and the Sci-Fi Museum that’s located next to the needle. If you’re into music or sci/fi or both, it’s a must see. There’s a combination pass for visiting both museums.
The Pike Place Market is a must see also, and if you’re lucky you’ll get to watch the unloading of fish which can be quite a spectacle.
If you have rented a car you could also go skiing (multiple resorts available), there are boat tours of the Puget Sound, underground tours and lots of other stuff to do. And if you’re not visiting Microsoft it could be worth just driving through (or visiting the Visitor Center in building 92) just to see how big their campus is. Building 92 also contains the Microsoft Company Store but you’ll need a pass to be able to shop, and hence it’s not available for anyone.