Yes, its all doable in a port day. Your walk to the train station in Monte Carlo will be a bit of a hike, but you would also have the option of taking one of the local buses (several of their lines go the station). As to Nice, if you really like to walk, its a good city to explore on your feet. You are talking about a few miles of stolling during your day, but for many of us that is just good exercise. There is no better way to discover a city than to walk.