Nice for one day -- September. Advice please.

Forget Castle Hill. There is so much else to see in Nice. Of course having a nice sit down lunch will use up a lot of your time for Nice. The local cuisine is Nicoise.
Mark Chagall Museum. The museum was built to house the specific paintings exactly where they are hung. A must see.
Musee Matisse.
Walk some of the neighborhoods. There are many and offer a taste of Nice. This is the best use of your time after the museums listed above.

Perhaps you will find a pharmacy on your walks. There are Galleries Lafayette in Nice; check there for French beauty products. Google Pharmacies in Nice to see if the products you want are available.
There is Old Town of course but it is touristy.
And the beach area, one part is the private beach Beau Rivage We stayed at Hotel Beau Rivage once and could use that portion of the private beach. The beaches in Nice are made of pebbles, not sand.
We loved staying in the area behind the famous Hotel Negresco, west end of beach.
Our last time in Nice we were there for two weeks, so much to see. We have been to Nice four times but have never visited Eze. I don’t like heights and sheer drop offs. Friends who spend months in Nice every year told us to skip it.
Villefrance is a pleasant town , set on a hill that goes down to the water. The only negative is that cruise ships moor there thus bringing crowds so we always checked to make sure no ships were in town when we wanted to go there. You are on a cruise so that is your reality. Perhaps you could see it after most of the other people on your cruise have left the ship or before they return. Leaving the ship at 7am I doubt anything much will be open. Maybe a coffee shop.

Along that coast is also exclusive Cap Ferat and the mansion/ museum with beautiful gardens, Villa Ephrussi which you can visit.
To the west of Nice, a visit to charming Antibes is also highly recommended. Musee Picasso.
If I were you I would cancel your lunch plans and see more of this beautiful area of France which was part of Italy until 1860, explaining the prevalence of Italian architecture and cuisine.

VilleFranche itself is a cute village but if you are having lunch elsewhere the town itself (and the chapel) are not that major.

It can be easier to catch the train into Nice instead, due to the number of people trying to get onto the 100 bus. The bus 100 is met up the hill whereas the train is at the port level. It will arrive in the port and you can walk to the old town or take the tram around Castle Hill. If you arrive by train, you can walk down Jean Medecin and through Place Massena or take the tram down.

If you arrive in the morning, you can head straight to the Cours Saleya market for a stroll and a coffee, and wander through the old town (the shops open a bit later).. Regarding Castle Hill, you can climb the stairs but there is also a free elevator to the top. The entrance is near the Hotel Suisse. A walk down will get you the same experience without the effort, unless you want the exercise

Were you looking for beach time? There are many potential beach clubs along the way, with the Beau Rivage being one of them. There is a nice beach (non pebble, more like large grain sand/gravel) in Villefranche-sur-Mer though I can imagine it can get filled up when a ship docks. A good long stroll along the Promenade is a nice thing as well. There are a number of museums you can hit (Chagall, Matisse, Massena, Modern/MAMAC, Beaux Arts) or cathedrals and the like.

It is possible to hit Eze in the same day, although transport can be time consuming. Bus 82/112 will take you to Eze village (there is a stop in VSM, up the hil or take it from Nice, at the start) or you can take an Uber/Taxi. You can take a taxi back down to VSM. Time is your enemy here but the fees will be less than what you pay on the tour. Unless you are have lunch or making perfume, a short visit is all that is needed (the view is the thing, here)

Thanks for replies.

We aren't interested in beach time -- we are mostly interested in looking at the water and we can do this with walking along the promenade.

Cancelling lunch is something to think about. The restaurant where we are booked is open from 12n-2pm for lunch. We aren't interested in lingering but we do want to have decent food since the food onboard is less than desirable.

Thank you for the train tip.

Great advice here. Thank you.

I had planned to meet a cousin coming on a cruise for one day in Nice. This caught my eye as an activity because Nadia combines food, neighborhoods, history and culture in her tour. My cousin canceled, but I think I’ll still go on the tour one of these days.
https://www.nicefoodandwinetours.com/

Just a quick comment: Musée Chagall is indeed great to visit (if you have any interest in Chagall), but Musée Matisse was a sore disappointment. There was very little of note inside the museum when I visited in ~2015-16. Reviews are mixed, accordingly.

And I would not cancel lunch: wandering around to find a suitable restaurant or takeaway will waste your time, and the restaurant you've picked is very well located for an old town visit.

There is a Matisse-Hockney exhibit at the Matisse Museum until September 18, which could possibly save this museum from being an utter waste of time and energy, particularly on a one-day stop.

With only one day available, I'd want to be sure all activities would be of interest. I like art a lot, but I don't care for Chagall at all. I was sorry I had spent time going up to the Chagall Museum. I recommend Googling for pictures of Chagall's work to be sure your reaction will not be the same as mine. There are many, many things to see in the area around Nice.

Thank you all.

Bets, that food and wine tour looks wonderful and would book this in a heartbeat if my dh was a wine person. That is right up my alley. Her website has given me some ideas, though.

As far as museums, we will probably skip since we were at the MoMA in May and saw a couple Chagall works, and there was a special Matisse exhibit on view called The Red Studio. We would like to see but pressed for time. I'm more interested in looking at the town and environs.

I think we are going to keep the lunch reservation -- I have romantic ideas surrounding French food. I've been to France once -- to Strasbourg -- and we had pizza. We are on a cruise where the food is not great and we want to have good meals in port as much as possible. I booked a terrace table which might have not been smart, depending on weather. We're going to Rome too and it is going to be frantic with no sit-down meal so this is our opportunity to eat a "real" lunch.

Thanks again for all of your suggestions.