Treat your wife and yourself to dinner at the Space Needle restaurant, make your booking so that you see the sun go down, spectacular views and a very nice meal albeit expensive, but you will remember it for all the right reasons.
Take a tour of the Seattle Underground for something different and if you change your mind about hiring a car, take a trip up to the Boeing factory. There is a large Mall/Outlets on the way where your wife could spend a few hours if Boeing doesn't grab her.