If you were hanging out in Eastern Washington more, I'd suggest the Grand Coulee dam. The tours are great, and the lazer light show at night is pretty cool. It's not too far from Spokane, but it is a bit of a trip. Also great for showing off the desert side of the sate, lol.
In Seattle, the Seattle Aquarium is pretty sweet, Pike's Place Market, Ye Olde Curiosity Shoppe, just hanging out by the piers (the aquarium and curiosity shop are on the piers, the market's fairly close by), are all great.
Oh, and while you're in Roslyn (not a whole lot to see), go down the road a bit and stop by Central Washington U and check out Ellensburg. Not that I'm advertising or anything. Not at all.