We took the bus from Aix to Marseille (I think that they are more frequent than the trains). Spent the day visiting the Chateau d'If and wandering in the old town of Marseille, ending with a bouillabaisse at a recommended restaurant and back to Aix (by bus) that evening. It's very feasible.

http://www.flickr.com/photos/mksfca/sets/72157624404539441/show/with/7743601548/

This year, 2013, Marseille has been designated a European Capital of Culture. You can google up a lot of news stories about it, but basically it means Marseille has received millions of euros to boost its tourist infrastructure and stage special art exhibits, public art, open events. I don't know if you are interested in history, but Marseille is France's second oldest city, and arguably one of its most dynamic cities of the future.

Isabel,

You are right, Sunday in July in Cassis will be a mob scene. In addition to the gobs of tourists, if the weather is good (which it will be) you'll have loads of locals from Aix and Marseille down to try to grab their 50cm-square speck of sand.

Personally, I'd add the day onto Aix and take the train or bus to Marseille - plenty there to keep you busy for a day even without the Cultural Capital events.

Completely different from anywhere else on your itinery, so it'll make a nice contrast for you.

- Kevin

p.s.: don't mean to denigrate Cassis by any means. Been there many times and love it, just not on a summer weekend.

Thanks for the replies. I think I will add the night to Aix with the idea being I'd take the train or bus to Marsaille for the day Sunday. I suppose that leaves me open enough that if I didn't like Marseille after a few hours I could always take the train to Cassis to check it out, but if I don't do that then on Monday, en route to Vence I could still stop in Casis. Might be less crowded on a Monday morning?

But thanks for the input.