Plus, the end of next month will be positively gorgeous (if my experience was any indication). The most perfect, continuous weather I've ever experienced was in Seattle. For over nine days, every morning it was around 67 degrees, and it warmed up to around 75 at the peak of the day. All nine days had no humidity at all (that's the real releif from Washington D.C.). *Sigh* those were wonderful mornings.

Oh, and I recommend seeing the Sound Garden. The band took its name from a sculpture in the area. Unfortunately, it's on NOAA property, and in recent years they've become really security concious (the terrorists will obviously target NOAA - in Seattle - first), so it's a little difficult to get in, but if you're there at the right time, you can get a visitor pass. I think your family would like it. It sounds very strange. Here's some Google maps of where it's located. The sculpture is merely a bunch of vertical pipes that turn to catch the wind, and create a very cool sound.

Other than that, I'm sure your daughter would get a real kick out of the Seattle Ducks tour, and would also like the troll under Aurora Bridge. Other than that, I can't really recommend many other sites that aren't well known, since I went there mostly on a tour of the sites of the Seattle music scene ("Grunge").