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Hi rhinowin, and welcome to MySwissAlps.
Tirano does not have much in the way of tourist attractions, and none of them are particularly close to the station.
There is the “Santuario della Beata Vergine di Tirano” (Sanctuary of the Blessed Virgin of Tirano), which in May is open Wednesdays, Saturdays and Sundays from 7:00 to 19:00. On other days in May, it closes between 12:15 and 14:30. It’s about 15 minutes’ walk from the station, so your visit would be pretty rushed. The website is only in Italian:
https://santuarioditirano.it/
There are no luggage lockers available at the Rhätische Bahn station in Tirano. You can leave luggage at the counter, for a charge of CHF 5.00.
http://www.rhb.ch/fr/service-souvenirs/contact/cont act-stations/detail/tirano
I would be inclined just to spend the hour in one of the restaurants around the station piazza, sitting outside if the weather is pleasant enough.
Alpenrose