Located within Marymoor Park (King County Park System), this 40-acre dog park is probably the best one ever! Gander loves this place - lots of open area to run, run and run some more. And then there are several hundred feet of access to the river where he could swim, swim and swim! After a couple of hours of being there, I would have a very tired dog for the rest of the day. Tired puppy = happy owner!
This park can get very busy, but the dogs (and their people) are mostly well-socialized and well-trained so it is usually a fun and pleasant experience. When you get tired of throwing the tennis ball, there are areas where you could sit and watch the dogs play and swap stories with other owners.
As much as Gander and I love this park, it has its down side. The area is not 100% fenced-in, so if a dog is not solid with his recall, it is possible for him to slip out into the parking lot. And with the area as big as it is, a dog could take off into it and it may take a while for the owner to find him. So this park is definitely more suitable with well-trained dogs that either stay close to their owners or come back reliably when called.
Despite the above, Gander and I always enjoy ourselves so much at Marymoor that I still give this dog park 5-star rating.
Directions: From I-5 or I-405, take State Route 520 east to the West Lake Sammamish Parkway exit. At the bottom of the ramp, go right (southbound) on W. Lake Sammamish Parkway NE. The park entrance is the next left at the traffic light. Follow the signs to the entrances to the off-leash area (2 large parking lots right by the off-leash area). $1 for all-day parking.