The stretch through Henty Road and Pile Road has some spectacular views back to Bunbury and the coast, with lots of corners winding through the valley. Recommend lunch at the Moody Cow Brewery, a stop at Gnomesville (if you've never seen it, it's an experience) and then onto Donnybrook which has a few Cafes and a Cidery. The rest of the route is mainly straight main roads through a rural setting.
Pretty boring from Bunbury to Forrestry road. Mostly straight high speed road (Highway).
Forrestry road into Harvey is pleasant although lookout for kangaroos in forrest area. Some farms so watch for gravel/mud. Mornington road is great in the lower section with some good corners, then it opens up a bit and again, kangaroos are the big hazard. Collie has lots of options for lunch then back to Bunbury via the coalfields highway, which has some nice sweepers along the way. Great lookout just as you go down Collie hill.