No stress. 10 days (which is not very long). Never Been. Want to Experience Charming Towns & Beautiful Scenery. [Food and history will be abundant, everywhere.]
To me, and I have only been to Italy twice since 2000 (one more coming up this April; two trips in the 1960s don't really count), I would suggest you consider No BIG Cities. No Rome; No Milan or Naples. Not that the cities are necessarily stressful, but you will have lines and crowds to go to museums that you would want to see (Vatican Museum in particular). Somewhat similar for the museums in Florence, too. IF you choose Florence, be aware that it will be crowded. What might that leave?
From north to south
* Italian Lakes (Como, Garda, Lugano)
* Dolomites and Alps
* Some smaller cities in or near the Veneto, such as Verona and Padova
* Emilia-Romagna (including Parma, Bologna and Ravenna
* Cinque Terre
* Pisa, Lucca
* Tuscan hill towns
* Umbrian hill towns
* Puglia
* Sicily
Pick just two, three at most. Or, just pick one - if Sicily or Puglia. You could easily work in 1 or 2 nights in Venice, if you focused other travel in and around the Veneto and/or Emilio Romagna.
If you choose Pisa & Lucca, or Tuscan hill towns, you could grab a little time in Florence and concentrate on churches (without lines or tickets) where you will delight in frescoes by Masaccio, Fra Angelico, Michelangelo, and Ghirlandaio, sculptures by Ghiberti, Michelangelo and Donatello, and sculpted terra cotta by the della Robbia family. For history, Florence would be incredible.
One other responder, Pat, mentioned the stress that you might encounter in visiting the Amalfi Coast. I don't know, never been there. But I have ruled out the AC as part of a 30 day trip we are planning to the south of Italy this April, for just that reason. See
https://community.ricksteves.com/travel-forum/italy/southern-italy-itinerary-assistance-request and https://community.ricksteves.com/travel-forum/italy/puglia-to-naples-itinerary-suggestions
Consider the weather; the south will be warmer (or hotter, if late spring). Mountains will still have snow in early spring. Does spring mean March/April? or May/June?
Read guide books. Consider a guided tour as others suggested, though you may find that those do not give you what you want - peaceful time in nature and peaceful explorations of charming towns.
Buon Viagge. Buono Anniversario.