Nice and Cannes

I would enjoy Cannes for the day, Nice doesn't offer much difference that would justify the journey. They're similar enough that it would make sense just to kick back and soak up the atmosphere without the hassle of making a journey elsewhere. For me the Cote d'Azur is all about relaxation, eating, drinking, people watching and enoying the sun and this can be achieved equally in Cannes or Nice.

While I don't disagree with JC, the train from Cannes to Nice is easy. So, if you find something you do want to see in Nice (such as the Russian Cathedral, the Chagall Museum, or the Matisse Museum), you can certainly go while in port at Cannes.

I don't agree with Rick's assessment of Cannes, that it's just about rich people and there's nothing to do there but look at the beachside glamour zone. While that zone certainly exists, if you go up toward the train station, or up the hill to the west of the glamour zone, you'll see other aspects of the city.

There are lots of attractive/charming beach towns along the Riviera. I agree that the reason to trek over to Nice for your port day would be because of specific attractions there, which are primarily art museums. In addition to those already mentioned, there are good museums of Asian art and naïve art. Those are not the only art museums in southern France, however.