If you have children in tow, the award-winning Imagine Children's Museum in downtown Everett is a must.  For more information, www.imaginecm.org  Not much else to do in downtown Everett for kids.  There is a large library within walking distance of the Children's Museum. 

During the summer, visit the small lighthouse at the beach in the nearby town of Mukilteo.  Year round you can also ride the ferry at Mukilteo as a walk-on passenger. It will take you to Whidbey Island and back.  The ferries run approximately every half hour.  Not much to do on the other side as a walk-on but no need to even get off, just the ride over and back is fun for families.  Take a walk along the rocky beach at Mukilteo near the lighthouse. Eat at Ivar's fine dining or fish bar across from the ferry terminal.