Hi. Marseille is a great city and for such a short time I wouldn't bother going beyond the waterfront and a few streets back. A trip up to the cathedral (Notre-Dame de la Garde) gives you a great view so it is worth a hike or taxi up there. If you want to go a bit further, visit a smaller town, go to a beach, then Cassis is around 30-45 minutes east of Marseille. Make sure the taxi takes you along the scenic D559 rather than the Autoroute A50.