Scenario One: You wish to hike in the French Alps at Altitude, but can go only in the second half of September. In France allmost all the refuges near Mt. Blanc or in the Vanoise will be closed. In the Mercantour Park, however, almost all refuges will be open.
Scenario Two: You will be in or near Nice for other reasons, and wish to start a hiking trip from there.
Here are the possibilities that I am aware of in 2023:
Hiking on the GR52 — see this page for itinerary details. All buses shown depart from the central bus station.
A). A one or two day trip: Take the train to Sospel from Nice (less than 1 hour trip). Consider overnighting there. Then hike the GR52 back to the Coast at Menton. This gives a spectacular descent from altitude that won’t be forgotten, with views up the coast to Monaco and beyond. It is a 6 to 8 hour hike. The initial section of the descent is stony and requires care. Return to Nice by train or stay an extra night in Menton.
B) A 5 or 6 day trip hiking on the GR52.: Take Ligne d’Azur bus #90 from the main terminal to Saint-Martin-Vesubie at 8:00 AM (all days) or 16:30 (M-Sa) ~ 1h 45m. Take the shuttle, if running, or a taxi, or walk 1 hour to Boréon. Possibly overnight in Boreon. Walk to the Madonna-de-Fenestre Refuge. Walk to the Vallée-des-Merveilles Refuge (Consider taking a guided tour there). Walk to Refuge L’Estive at the Camp d’Argent. Walk to Sospel. Walk to Menton as per (A).
C). A 6 or 7 day trip: Take Ligne d’Azur bus 92 departing at 10:17 (check) but only during the school vacation period. Trip of about 1h45. If this is not running, then bus 91 departing at 8 am to Isola Village, and a taxi from there to Isola 2000 (perhaps 15 Euros). Possibly overnight. Walk to Boréon. Continue as per (B).
D). A 9 to 11 day trip: Take Ligne d’Azur bus 91 to St-Etienne-de-Tinée (trip about 1h45). Possibly overnight. Climb from there to. the Refuge de Rabuons. Walk to Refugio del Laus in Italy. Walk to Sant’ Anna in Italy. Walk to Isola 2000 or on to Boréon.
E). A 11-13 day trip, expensive unless group of 4. Take the bus as per (D) above to St-Etienne-de-Tinée. Hire a taxi to cross the Col de Bonnette (the highest pass in Europe at 2,700 meters), an exciting ride, to arrive at Larche (94 km one way). Overnight. Follow one of the routes to the Refuges du Vens and Rabuons — 2 extra nights.
Hiking on the GR5 —see this page for itinerary details.
A) A 4 day trip: Take Ligne d’Azur bus 91 to St-Saveur-sur-Tinée (1h15) (Depart 8:00 every day 16:30 and 18:00 M-S. . Walk to St-Dalmas- Valdeblore and overnight. Walk to Utelle. Walk to Levans or Aspremont. Walk to Nice.
B) A 6-7 day trip: Take the Ligne d;;Azur bus 91 to St-Etienne-de-Tinée or Auron (1h45). Possibly overnight. Walk to Roya. Walk to Lognon. Continue as in (A).
C). A 9 day trip. Access as per (E) above. Night in Larche. Continue to Bousiéyas. Continue to St-Etienne-de-Tinée. Then follow (C).