If you are on the CTM bus you will blow through the argan forest at 100 km/h without stopping. That is fine. But if you have a private vehicle — whether a transfer or your own rental — the road between Marrakech and Agadir is worth slowing down for.
The most popular stop is an Argan oil cooperative run by local women somewhere around the Chichaoua–Ait Melloul corridor. These cooperatives are the real thing: women cracking nuts by hand, pressing cold-pressed oil on site, and selling it at honest prices without the markup you find in Marrakech medina shops. Allow 20–30 minutes and expect to buy at least a small bottle.
The second worthwhile detour is Imouzzer des Ida Outanane, a small Berber town set in a canyon about 60 km north-east of Agadir. The road up is a narrow switchback that feels more dramatic than the actual elevation warrants, and there is a waterfall (Cascade d’Imouzzer) that flows most reliably in spring. It adds about 45 minutes to the journey but feels like a different Morocco entirely — quiet, agricultural, and largely tourist-free.
Further afield, a private driver heading south can loop through Taroudant, the walled city sometimes called "little Marrakech." The souks are calmer, the kasbahs are intact, and the local market has none of the tourist-price theatrics of Djemaa el-Fna. This turns the transfer into a half-day journey — perfectly manageable if you are not in a hurry.