Short Day Trip by Train - Train des Pignes?

Entrevaux is a very picturesque medieval town. Probably very touristy if you go on a nice day. I went on an exceptionally rainy day, and I was the only tourist in town. The train ride was scenic even in the rain. It would be a nice day out from Nice. I don't know that it works as a half-day trip. I can't find the train schedule online (I'm sure it's out there), but I remember that the train doesn't run very often. You'd need to see whether there's a return trip from Entrvaux that gives you some time in that village but doesn't keep you away from Nice too long.

The train doesn't depart from the Nice Ville station, but from a separate station not too far away. The fare on this special train is rather expensive.

Both St-Paul-de-Vence (very touristy and quite small) and Vence (not especially touristy and larger) are accessible by bus from Nice. They don't have rail service. The two are linked by bus and can be visited on the same day, but I wouldn't attempt that in just half a day. Better to pick one. I much preferred Vence, but the Gallerie Maeght (modern art) in St-Paul-de-Vence is definitely worthwhile.

There are actually two trains: the regular train and a special steam train (connecting, with joint tickets available). Entrevaux is a popular destination. It probably is not a half day trip. It departs from the Gare du Sud, at the Liberation tram 1 stop, north of the main train station.

Another possible train ride is the "Train des Merveilles". This goes all the way to Tende, but other stops are possible. It departs from the main station, with the 9:08 am departure having the narration. It is rather long to Tende, however. Sospel is the first stop but may not be as interesting.

If you want a half day trip you might just consider the standard coastal train from Nice to Menton and back -- or even just as close as Villefranche-sur-Mer or Juan-les-Pins (easy sandy beach access).