Just need to store a suitcase for 5 days in Nice while I travel in Italy. Are there lockers at the airport or somewhere in town? I wrote to the hotel I am staying at for the night in Nice before I fly home but have not heard back. The suitcase is US size carry-on but too big for Ryan Air.
Is your bag too big to check in on the flight?
Try Bag BNB
https://bagbnb.com/luggage-storage/nice
There is bag storage at the airport, at the train station and at private spots throughout town (including some of those above at the link) at 5 to 14E per day. The official airport station is the most expensive option . If you are returning by train, you might consider leaving your bags at the main Nice Ville gare. https://consigne-avignon.com/left-luggage-nice/ although the private concessions (shops and hotels, generally) are going to be cheaper.