I you have the possibility to change your STAYINGlocation ...I would recommend it... as the train station area is not the safest in town(there is however a good Ibis in the sation premisses) and try to book a room @the Novotel Marseille Vieux Port,or the Residence or the Mercure Bauveau on the Vieux Port.
Yous hould expect a sunny day even if the cold mistral wind is blowing.
As for the sightseeing;
-go for a unique view and a visit of the Bonne Mere Cathedral is a must.
-hicking along the Vieux Port, the PHARO palace park ,the Catalans beach and the Corniche up to... the Pointe Rouge resort;a good opportunity to picnic on the beachside or have a lunch ,the best way to catch the vibrant way of life of marseille.
-visit the old town of Panier and an exhibition at the Vielle Charite Museum.
-a day trip to Aix en Provence and/or Avignon is a must.
-...a visit of Cassis ,the upscale hidden gem,and of course a wine tasting of the rare cassis white wine at the Bar de la Marine
-a mini cruise to Chateau d' If island and ...rediscover Duma's Count of Montecristo... and finally have a drink at the Sofitel's bar