The Valli dei Mulini (Vallone dei Mulini, singular) are actually two deep gorges where abandoned mill buildings from centuries past have been taken over by Madre Natura and turned into a nature preserves, often looking like they deserve to be in one of the Lord of the Rings stories.
Further east, as the road comes upward from Naples toward the Amalfi Coast (just east of the town of Gragnano), there are abandoned mills alongside the Via del Presepe, winding through its millstream gorge. The Via del Presepe is a very narrow, cobblestone paved road, so don't drive it with an overly wide vehicle.
Today, there are walking tours of the Vallone in Sorrento and you can easily drive streamside to see the mills near Gragnano. Bring your cameras, water colors and leaf folios...