I've been myself, in an attempt to drop off a resume (I live in the area).
I do not recommend visiting. You get a tour if you're invited to a tour, you can't just show up and expect one (or even schedule one). It's also a big office building, not a theme park. There is a dragon in the entrance but that's it, and without knowing anyone you aren't going to get farther than that.
You have to remember that this is people's workspace, not a museum. The folks there do not want to be bothered by fans of D&D or MtG, they want to do their work to the best of their abilities.
It's also in Redmond (not Seattle proper) where there is nothing around particularly tourist-y, so you'll be driving there to see the entrance of an office and that's about it.