Exit the medina at Bab El Ansar and head downhill a few metres until you cross the river. Turn right after the bridge and follow the path on the eastern side of the river, Oued Ras El Maa. The route has been prettily landscaped, and meanders alongside the water. There are spectacular views of the medina, and it all makes a very pleasant downhill saunter of around half an hour. The path meets Ave Allal Ben Abdallah, where you can hail a taxi to take you back to the medina.
The Talassemtane National Park starts just outside Chefchaouen and has numerous hiking options. If you're camping, you must register at the park entrance, where there's also an informative eco-museum.
The Bouhachem Regional Nature Reserve lies between Tetouan, Chefchaouen and Larache, and has a number of treks of various lengths visiting local villages and exploring the mountains, forests and waterfalls. There are a several gîtes (basic homestays or hostels) for overnight stays. The park is exceptionally beautiful and covers an enormous area of 80,000 sq km. Designated a Site of Biological and Ecological Interest, it is, along with the nearby Talassemtane National Park, one of the core areas of the Intercontinental Biosphere Reserve of the Mediterranean shared between Andalusia (southern Spain) and Morocco. The forest has various species of oak, maritime pine and cedar. The park is home to an important number of birds (99 species), mammals (32 species including the Barbary macaque) and reptiles (17 species).