We had a last minute request from our son to take him and his three year old with us camping last weekend somewhere near Seattle. A search for reservation spots did not yield anything. We discovered Camano Island State Park
near Marysville WA had "first come first served" campsites. Maria and I went there on Thursday afternoon and found there were spots available and picked a beautiful large site (#72) in the upper loop. Son and grandson arrived on Friday evening.
What a great spot! The woods around here are full of interesting things to see, and the island has plenty of scenery. I've posted a few more pictures here:
Camping at Camano Island State Park - a set on Flickr