Visit the Light House. Visiting hours are 12pm-5pm weekends & holidays April-September. It right on the beach so take a stroll afterwards. Best of all though, tours are FREE!
 

Ride the Ferry! Its $4.65 for adults, $3.75 for kids and just $2.30 for seniors. Walk on and go for just the rid and the beach on the other side or drive (just $7.85 for car & driver) and explore Whidbey island (including Fort Casey). 
 

Hit the Diamond Knot. Great food, better beer. Free peanuts and you get to just throw the shells on the floor, it’s very satisfying. 
 

Hike Japanese Gulch. Head East on 5th St from the Mukilteo Speedway (just north of the ferry landing) just past Prospect Ave there is a parking lot on the right. The trail is a total of 14 miles but just do as much as you like. Its very pretty back there.
 

See some Airplanes (technically in Everett, but still very close!). There are three places you can go. The Boeing Plant in Everett.   Future of Flight Museum. or The Flying Heritage Collection (Adults $12, Senior/Military $10, Youth $8, 5 and under free). This is Paul Allen’s personal collection, consisting of mainly combat aircraft from WWII. These planes are flown during the summer and are very fun to watch.
 

Visit Traxx. Traxx is an indoor (and outdoor) raceway featuring extra fast go-karts, Ages 11 and up can do a standard race for $20 or you can climb the rock wall, visit the bouncy basketball house, stick with the Velcro wall and more! Great place to go on a rainy day!  
 

Go to the Rose Hill community Center and see whats going on for the day. They have dance classes, painting classes, workouts and more!
 

Go Golfing! Award winning course at Harbour Pointe Golf Club. It is a picturesque, beautifully designed course with Mountain and Sound views. Prices range from $15-45 depending upon day and time and golf carts and just $15 a day.