The short answer
The Ourika Valley for river air and waterfalls about 1 hour 15 minutes south; the Agafay desert for a camel ride and camp dinner about 40 minutes away; Terres d'Amanar in the foothills for zip-lines and pony rides; and Essaouira for a beach, ramparts and fresh fish. All work as private day trips, back in Marrakech by evening.
In more detail
The Ourika Valley climbs into the High Atlas along a clear river, past Berber villages and riverside cafés with tables set over the water. Our Ourika day stops at a women's argan cooperative and a Berber home. The walk to the first Setti Fatma waterfall is a guided scramble over rocks, about an hour each way, so it suits older children; younger ones can paddle and picnic lower down.
Agafay is a stone desert of rolling hills, not dunes, about 40 minutes from the medina. Our sunset trip gives a choice of a relaxed camel caravan or a guided quad circuit, mint tea at sunset and dinner in camp under the stars. It is an easy outing for families and a gentle taste of desert without the long Sahara drive.
Terres d'Amanar, about 40 minutes away at Tahanaout in the foothills, is an outdoor park with zip-lines, a via ferrata, archery, pony rides and trails, built for active families. Essaouira, about 2 hours 45 minutes west, adds a windy beach, ramparts and the harbour, and suits very young children who need downtime between sights.
For the youngest, a short Palmeraie camel ride on the edge of the city is often enough adventure for one day.