I love the Seattle Ferries--long trips thru the San Juan Islands (you can take a car and continue to Victoria--Buchart Gardens is a real treat!!)
Mt. St Helens volcanic "blow-out" is pretty amazing
Mt. Ranier is visible form nearly everywhere around Seattle--great hikes and nature
Seattle Pike Place Market--YUMMMMY FOOD great produce!
Wineries outside of the city--St. Michele, herb farms and some of the best food you can get anywhere
Then there are the threads here about Boeing--what 5 yr old wouldn't love that stuff???
There are the typical tourist things at the waterfront, too--like Imax, harbor cruises, aquarium and that sort of stuff