Nice, brief tour of the city followed by an extended on-your-own visit of the aquarium. Prompt shuttle back to ship. The aquarium is really extensive. A brief city tour is really all that's needed here, in my opinion.
We visited Aix-en-Provence on our own but used the transportation there and back provided by the cruise line. So it was a cruise line excursion, but we were on our own, which worked well. Aix-en-Provence is relatively compact.
We simply got a taxi at the port and used that to tour around... there are a zillion touts right where you disembark. That worked relatively well.