One day in Nice.

I recommend catching the bus (81 or 100) or the local train or take a cab, to the Villa d'Ephrussi and Rothchilds gardens. It is about 8 km east of Nice. There is a fabulous garden and a great home to tour. They have a small cafe with nice (a bit expensive) salads; I do splurge when I go there and have lunch in the garden. Rick has a walk from the gardens into St jean cap Ferrat. short walk and sea views.

If you wish to stay in Nice, there are great museums, the Palais Lascaris is great (in the Old Town), and the market in the Cours Saleya is also lovely. Just walking along the water is fabulous.

Enjoy France.

Obviously it depends on your own personal interests. Ours includes art, so we visited the Chagall Museum and enjoyed it. The audioguide is included and was a great help to observe more details in the paintings.

Places to walk: along the beach on the English promenade, up to Castle Hill (park), through the old town (possibly a market).
Places to see: Chagall, Museum of Art and History (Villa Massena), Orthodox Russian Church. There is also an Archeology museum if you won't have a chance to see any other ruins.

Not miss depends on what interests you most.

Do you have Rick Steves Mediterranean Cruise Ports? It not only has lots of information on what to do with your day in Nice, but should have information on most (perhaps not all) of your other cruise ports. The book is also very comprehensive about how to get around on your own to and from the port, so you can see what's involved and decide if you want to use the ship's excursions or do it on your own (on your own is easy in Nice, but if you want to go further afield, say to Eze or St. Paul de Vence, you may want an excursion).

As a taster, here's Rick's list of highlights for Nice (scroll down and click "At A Glance"): https://www.ricksteves.com/europe/france/nice

Do get hold of a comprehensive guidebook so you have more complete information on the attractions in Nice. There's sonething there, or nearby, for just about any taste. For example, I really liked the Naive Art Museum and the Asian Art Museum. Afterwards, I was told that there's a nice garden near the Asian museum. I'd definitely want to spend some time wandering around the Old Port.