bushrat,
Will you be picking up a car at the airport? If you are looking for a beach town, we like Bandol, about an hour east of Marseille. It has a lovely harbor area, a small pleasant beach, a classic farmer's market, nearby wineries to visit, and a pleasant Mediterranean ambience. You can stroll along the beachfront promenade. There is a carousel and casino there as well. I don't know Cassis, but it has more beach than Bandol, and is not far away.
Inland, St. Remy de Provence is a good choice. We really like it for it's size, charm, Glanum Roman ruins, historical art background,
and it is not a long drive to many other charming villages.
Paris has so many things to see and do it is hard to even begin to list them, so I won't even try.
I hope your 11 days are days on the ground. If they include arrival and departure days I would drop one location. If you have 11 full days, I would spend them in this way:
Day 1-Arrive Marseille, pick up car, drive to Bandol-Sleep Bandol
Day 2 and 3-Bandol and neighborhood-Sleep Bandol
Day 4-Drive to St. Remy (half day) and explore town-Sleep St.Remy
Day5 and 6-Explore towns and area around St. Remy-Sleep St. Remy
Day 7-Drive to Avignon, turn in car and take TGV to Paris (Gare de Lyon)-Sleep Paris
Day 8, 9, 10 and 11-Paris-Sleep Paris
Day 12-Leave (If return home is on day 11, then "Sleep Paris": is just Day 8, 9 and 10 in Paris. These are all full days in Paris.
(If you are not renting a car at all, the beach plans need a major adjustment from what I suggested.)
Have a great trip!