Adjacent to the ferry terminal, Mukilteo Lighthouse Park is home to the scenic Mukilteo Lighthouse (425/355-9602, www.parks.wa.gov [1]). Built in 1905, this red-and-white beauty is still functional and, along with its restored outbuildings, is open for tours noon–5 p.m. Saturday and Sunday April–September, for a token $1 donation.
The park also has a popular beach for sunbathing, launching boats, and casting lines from shore into the bountiful Possession Sound. Because of consistent winds, the park is also a favorite with kite flyers and windsurfers. The water around here is treacherous, though, so it isn't recommended for casual swimmers.
Links:
[1] http://www.parks.wa.gov