I actually have this philosophy that you should generally go to your furthest point (from the ship) first, and than work your way back towards the port during the day. Why not take the train to Monte Carlo and see how it goes with time. Since its all on the same train line you could stop at Nice on the way back. Save Cannes for last since the city is all within walking distance of the tender pier, so you can walk around until its time to get the tender. As to the Grand Prix course, I am sure you know that its on the regular city streets. The course is very walkable and winds through the central part of the city right on the water. Unless you are there near race week, there is nothing to see that would give a hint they run a race on these streets. I have driven the streets, but with all the traffic I think I got my Peogeot rental car up to about 10 MPH
Hank