Thanks, Hank, for the insight.
Will definitely verify with Hertz that the airport location is open. I have contracted to return the car, after hours, to the downtown Cannes location (147 Rue d'Antibes). Perhaps, I should verify that too.
So it seems like a good plan would be to get my rental car as early as possible from Cannes airport, make our way to Nice to see St. Paul de Vence and the flower market, then make our way to Monte Carlo Casino, which opens at 2pm, anyway. Then around 4pm, I can make my way back to Cannes, drop off the rental car, and walk back to the ship, which departs at 7p. Does that sound practical?