Sestri Levante or Finale Ligure day trip from Milan

Look at Noli near Finale Ligure too.

Of the 3 bases,
Treviso is the closest to the coast. Lido di Jesolo is the most important beach destination near Treviso.

Florence is the second closest to the coast. The Versilia area is the most popular beach destination in this area (Viareggio to Forte dei Marmi). Castiglioncello, just south of Livorno is also nice.

Milan is the farthest from the sea.

Thank you Suki and Roberto. Will look at those two places :)

Be sure to manage your beach expectations, there are fine beaches in Tuscany and near Treviso, but if you live in the US you have better beaches closer to home. For instance, the beaches on the islands and peninsulas around Venice / Treviso are no match to the N Carolina Outer Banks.

Thank you Balso for your advice :)

Lido di Jesolo and the Versilia area are very similar. An urban environment with flat sandy beaches like Miami (although the sand is not white like Florida).
Lido di Jesolo: https://images.app.goo.gl/Q67qZvY5aX598wW4A
Viareggio: https://images.app.goo.gl/GPe6jdb1doZQ3ucx6

Castiglioncello is a pretty town with more cliffs, but clearer waters.
Castiglioncello: https://images.app.goo.gl/T7bR1wqVgNbWCAtD6

If you want white beaches you can go just south of Castiglioncello at Rosignano.
https://images.app.goo.gl/o5BazxmUYProFYBk8
The white beaches are due to the bleaching effect caused by the nearby Solvay chemical plant, which produces chlorine, sodium bicarbonate, and other products that over the last century have created an artificial Caribbean landscape.

Thanks Roberto, Ill look into Castiglioncello :)