There are several things that would be fun to do, but I'm not sure if there are luggage storage facilities at the cruise ship terminal. You could walk to the famous Pike Place Market
, or take a taxi to Pioneer Square for the Underground Tour
of city streets that were under the current streets.
If Royal Caribbean is offering a post-cruise tour of Seattle, you may want to consider that. They take you and your luggage on a tour, which ends at the airport. It's a very convenient way to do touring, transfer and luggage handling all at the same time.