I generally keep my dog on leash at Walnut Creek, mostly because in wooded areas a squirrel becomes much more attractive than the sound of my voice calling my dog to come, but also because it is a hot spot for trail riders. Hikers and their dogs need to be on alert for passing bikers. However, the park is very clean and has lots of trails to walk. The trails are marked, but if you're not paying attention, you can get turned around.
It also seems to be one of the parks less frequented by dog enthusiasts, and is big enough that you are not constantly running into other hikers and their pets.