I agree that Leavenworth is worth a stop and that there's not much to see in Roslyn. If you stop in Leavenworth, the best brats are at Munchen Haus.
In Seattle, the space needle at dusk is awesome. You can check out the views, then stick around for the sunset. Just beautiful! The EMS (music and sci-fi museums) is right next door and fun too.
A ferry trip out to Bainbridge Island is cool if you've never done it, and gives you an awesome view of the Seattle skyline.
Go to Fairmont, see the troll under the bridge, check out the local shops.
Pikes place is fun, and the original Starbucks is across the street.