France: Regional Train Tickets Stopover Rules

With a ticket bought on the blue (IIRC) TER ticket machines, yes, but not with online tickets. Has to be within the same day (calendar day). And you must validate/stamp (composter) the ticket in a yellow stamping machine the first time you use it.

TER tickets are "valid for the selected date of travel," not a particular departure, so I believe you're allowed to break the trip. Note that buses are slower but more frequent, may stop closer to your destination, and are also cheap for either single tickets or a €5 all-day ticket.

Thanks. I had planned to wait to buy the tickets from the machine when we are ready to take a day trip but could not remember from past trips about getting off and on again, etc.
Yes, i did read in RS snapshot Nice & French Riviera about the buses. Have not used the bus before but will consider that as an alternative to the train. Do the buses leave from near the Nice train station or another location in Nice?

There's a bus info and ticket office across the street from Nice Ville train station, but more of the bus stops are along the water. Currently bus #100 in the direction of Villefranche and Monaco leaves from the inland/north side of the port, by the Port Lympia stop on Tram 2. But also see https://www.lignesdazur.com/en/department-bus-lines/108 or confirm locally, since they may add another, more central stop, now that tram construction is done.

Thanks Laura, I will check the bus schedule and departure locations.