Immediately south of Mundaring is Beelu National Park, part of a continuous swath of forest that includes Kalamunda National Park. There's a handy campground (adult/child $11/7), well positioned for the Bibbulmun Track and Munda Biddi Trail, which pass nearby, and with access to a viewpoint over the Mundaring Weir. Stop by the Parks & Wildlife Perth Hills office for maps and info.
From November to April, kick back in a deck chair at the open-air Kookaburra Outdoor Cinema, just across the road from the park campground.