Our train will get in from Nice around 12 noon;; then we were thinking of touring the Palais des Papes and gardens. Will an afternoon be enought time or should we do that the next day.
I ejoyed visiting the Chartreuse du Val de Benediction in Villeneuve-les-Avignon. It was founded in the 14th century by Pope Innocent VI in his former Cardinal's Palace. It is pretty close to Old Town. The Palais des Papes and gardens isn't that big. Sometimes they have events there. There is a big Market Hall in Avignon. You also can visit the Pont du Gard a Huge Roman Viaduct nearby.
I spent 4 days in Avignon in 2024. Didn't find it particularly interesting in itself. It does have a nice art museum, the Calvet. However, day trips to Pont du Gard, Gordes, and Roussillon more than made up for it. If you like Roman architecture, the intact Pont Julien near Roussillon is stunning. I suggest using the 2nd day to do a day trip to one of the places I have listed, or to Alres, Uzes, or Neims. Arles and Nimes are more interesting than Avignon, in my opinion.
An afternoon is plenty of time to see the palace and gardens. Honestly, I know it’s hard to NOT go in, but you can absolutely skip going inside the palace. There’s literally nothing inside.
I hope you plan of seeing Pont du Garde or utilize Avignon as a day trip base for the following day.