Your post was so new, it came up on the main Travel Forum page, so I wasn’t looking in a particular country category. And when I first saw your post title, and started reading it, all along I was thinking of the eastern side of the Adriatic. Then it occurred to me you were envisioning starting south of Venice, then continuing south, on the Italian side.
While I haven’t yet done that, including visiting the tiny, separate country of San Marino, we once rented a car in Venice, then drove north a bit, then east, and then south. We turned the car back in in Venice. If you haven’t seen Slovenia and/or Croatia, they’re definitely worthwhile.