Admission
There is a $10 fee to visit each state park (usually accepted in cash or check) but at $30 you can get the Discover Pass which is an annual pass for all WA state parks.
When to Go
Any time of year(however, some state parks are closed in the winter)
Things to Do
Ferry ride, exploring the trail in the woods, enjoy the forest, throwing pebbles in the water, etc.
Tips
1) Visiting Mukilteo Lighthouse Park before or after the ferry ride can be nice.
2) Ivar’s is located right next to the Mukilteo Ferry Terminal, so if you have time a Clam Chowder to-go will be a great snack on the ferry ride. The snack bar inside the ferry also sells Ivar’s Clam Chowders.
3) Whidbey Coffee (which is actually headquartered in Mukilteo) is located in many locations throughout Whidbey Island (there is a nice cafe location in Oak Harbor) so stop by for a cup of local coffee.