Marseille is well placed for day trips in every direction. Aix-en-Provence, the elegant fountain-filled university town, is 30 minutes by train. Cassis to the east, a small Provençal port at the end of the Calanques, makes a perfect coastal day out — boat tours of the calanques leave from the harbour. The Camargue to the west, with its flamingos, white horses and salt pans, is reachable by train or car for a longer day. Avignon with its papal palace is 90 minutes north by TGV. Arles, in the Camargue, has the best Roman remains in southern France.
For walkers, the trails into the Calanques National Park start from inside the city and the south-eastern suburbs, so a full-day hike along the coast to Cassis is doable as a day trip (and the train brings you back). The hilltop village of Aubagne, birthplace of Marcel Pagnol, is a short trip on the Aubagne tram and a good base for walks in the Garlaban hills he wrote about.