Between Nice and Marseille

Well its slightly the wrong way but I would suggest Villefranche sur Mer or any of the little towns between Nice and Monaco. I wasn't particularly impressed going the other way until nearly Toulon but the cliff get a bit steep by there.

Bandol, Sanary sur Mer or Cassis

I really like Cassis.

Cassis is fantastic but it's really close to Marsielle so if you were looking for something more "inbetween" it might not be for you. But the trip still isn't that long between Marsielle and Nice so I'd definately go for Cassis. There is a wonderful "hostel" with two private rooms up on the hill that serves breakfast and has a pool overlooking the town and sea, and can be as low as 50 euro per night for the private room. I don't remember it's name but it's the only hostel actually in Cassis (there's another one outside the city). I loved my stay there and think it's a fantastic deal. It was like staying in a really nice person's vacation home!

It is the other way but I second Villafranche.

Another vote for Cassis and Bandol.

One of my favorite seaside towns is on the road between Nice and Marsailles and is called St. Maxime's. It is a perfect size town for me and has wonderful shops, restaurants and a great market (don't remember what day) but has a nice beach with a carousel in the park on the beach. I have stayed there a number of times and it never disappoints me or the many friends I have taken there. The drive along the coast is beautiful.

Assuming your desire is to break up the trip and you are looking for a town, not necessarily a sandy beach, the choice is clear: St Tropez. It is a wonderful town, very colorful. It is about half way between Nice and Marseille. Beautiful harbor, wonderful streets to walk and shops to visit.