Avoid it like the plague, shittiest place in all of France. You're better off cancelling and rebooking to the Riviera instead, better vibe, better architecture, better everything. If you really want to blow your girlfriend's tits off you should do one of the following:
1. Lyon > Annecy > Geneva (I know, not france but its close enough to get to from annecy for a day or two) > Chamonix
2. St. Emilion > Dordogne river tour to Bergerac > Beynac and La Roque > Sarlat > Perigeux if you don't mind it being out of the way.
3. Land in Paris, rent a car and immediately leave for Rouen > Honfleur > Beuvron-en-Auge> Caen > Mont Saint Michelle > Rennes
4. Montpelier > Nimes > Daytrip to Pont du Gard > Arles > Daytrip to Marseille if you really need to visit that shithole
5. Toulouse > Carcassone > Narbonne > Beziers > Montpelier
6. Paris and immediately hop on a train to Nancy > Strasbourg > Selestat > Colmar
Look up videos and photos of the stops on #4 and you'll see just how much of a scam marseille is given everything around it.