I've taken multiple family members on the Underground tour and all have found it quite interesting and enjoyable... It's especially great if you go on a rainy day -- perfect thing to do to introduce them to Seattle's past and, if you haven't done it, you should go too.
Other areas we've enjoyed include Port Townsent, Ruby Beach (on the coast), Bellingham and of course, Vancouver.
But, if you want to stay within the city, start down with the Underground... Great stuff to see down there and then, when you're done, go have a beer at one of the old ale houses down there.
more