The Russell Investments Center is a privately owned office building downtown, but if you have a friend who works there, or happen to pay a visit during a busy weekday lunch hour, you may find that you're able to get up to the 17th floor, which has one of the most unique spaces anywhere downtown-- a grassy outdoor park and observation deck, on the 17th floor of the building. There's a little lunch counter up there, and quite a few places both indoors and outdoors for folks to sit and eat lunch.
From the edge, you can look out over the railing and have great views of downtown, the waterfront, and on a clear day, the Olympic Peninsula over Elliott Bay. The view, particularly of Pike Place Market and Downtown, is a bit different than the ones you can get from most public places, and the grassy field on the roof of a downtown skyscraper definitely feels out of place-- in fact, the landscape of dry grass is one you'd be hard pressed to find almost anywhere west of the Cascade Range. It feels a bit surreal, but is definitely worth a quick visit if you're able to get up there.