Not sure what you intend for Fall 2014. If it's around Sept. 21, then beaches are feasible, if closer to Dec. 21, beaches are not feasible. Actually these past few weeks Italy has had some major snowfall in various parts of the country.
The weather in Central/Southern Italy is very similar to Northern California's, at least temperature wise, since Italy is not as dry as here in California (especially so far this year). Northern Italy is more similar to Oregon, temperature wise, but with less rain than Portland, since the Alps act as a shield against many weather systems coming from the Atlantic.
If you go early in the Fall (Sept/early October), the Dolomites are good. You could fly to Venice and after visiting the city a few days you could rent a car from Piazzale Roma and venture to the Dolomites (I recommend the Val Gardena, such as the town of Ortisei or Selva etc.) . In NorthEast Italy you could visit Lake Garda (nice beaches and mountains dropping onto the lake), Valpolicella (wine region), Colli Euganei (more wine and spas), the cities of Verona, Vicenza, Padua. Ravenna and Ferrara are close by as well.
Lake Como is closer to Milan (about 2.5 hours east of Venice) but 3 weeks is actually quite a bit and you could visit all of Northern Italy in that time frame, including Lombardy and Piedmont with all the Alps and all the major lakes (Garda, Como, Maggiore, Orta). With a car you'll have more flexibility so it will be a good opportunity to visit the small towns and countryside.
If your husband is interested in sea (not just lake) beaches, near Venice I recommend Jesolo. Croatia however has better places on the North Adriatic coast. If you decide to visit there, inquire with the rental car company because there are rental restrictions which may prevent you from taking the car across into the Eastern European countries (lots of car burglaries there). Maybe Croatia won't be a problem, but still inquire (just don't drop the car off to a different country from where you pick up, because the drop off fee would be exorbitant).
On the Italian side, the Northern Adriatic coast is nicer around Mt. Conero, near Ancona. The Rimini to Cattolica area is more geared to entertainment (not so much nature and beauty of the sea). The crowds mentioned by Ken will be gone in September, when the area becomes a beach vacation spot primarily for retirees. Riccione is the nicest town in the area IMO.