I wanted to like this park but the quadrants are so small and only one is off leash. Also, the northeast quadrant has a creek and my dog got giardia from drinking out of it...cost us a lot of money to get the house professionally cleaned and pay for her vet bills. So bummed! Don't let your dogs drink the water!
Pioneer park is not primarily used for mountain biking, although some do run the trails. The northwest quadrant is basically level, with lovely trails meandering through the woods. A horse trail runs along the eastern edge of the park. It is it acceptable to have off leash (but under voice command) dogs in this quadrant. The northeast quadrant is leash only, has some hills, and has access to Lake Washington. The southeast quadrant is also off leash, is somewhat hilly, and all trails are open to horses. The southwest quadrant has a stable and retail stores. Great park, we go daily!