There is a great one in Margaret River and around there. I always took the short one however you could do a longer hike there, and the part I did was fairly easy, running along the river just outside the township of Margaret River.
There are some great trails through John Forrest National Park. I ride through there quite frequently, it's a beautiful place. The Eagle View Walk Trail is a 15km bushwalking circuit that will take you to some of the park's less explored parts. Well worth the trip out to Mundaring!