Bess, I think you could have a wonderful trip to either Croatia or Italy. I would recommend that you also post your question in the Italy forum, to get other people's suggestions and hopefully hear from people who were in Italy at that time of year. In Italy, I would recommend getting off the beaten tourist track and focusing your time in Le Marche (Urbino, Ascoli Piceno, with some beach time in Senigallia). Look up all these places (and the ones mentioned below) on Trip Advisor.
For Croatia, consider flying into Split and spending a couple of nights in or near Diocletian's Palace. (We stayed at Base Rooms, in the palace, simple but nice and great location.) Then take a ferry to the tiny island of Vis. This is not as well-known as Hvar or Korcula but equally beautiful. We stayed in the main town (Luka), which is very pretty. But even more appealing is the little town of Komiza, on the other side of the island, reachable by bus. Absolutely gorgeous. I would stay there if possible. This would be a great place to be lazy for maybe 5 nights. You can take a boat excursion to the "blue hole" of Bisevo, which is fun. You can do a little hiking on the island ... it's very small, and does not get many tourists. We loved sitting at the harbor in Luka at sunset with a glass of wine and watching the yachts dock.
Then ferry back to the mainland and visit Plitvice Lakes, again by bus. We haven't been there yet, but have heard that it's amazing. Stay overnight here, and then make your way down to Dubrovnik. To avoid crowds, don't stay in Dubrovnik. Instead, keep going on the bus about 15 minutes south to the beautiful little village of Cavtat. Our favorite B&B here is Villa Andro, but they book up quickly. You can visit Dubrovnik by bus, or by boat (about 40 minutes). Preferably, visit Dubrovnik on a day when no cruise ships are in port (check this website to see how many ships & passengers are scheduled for each day).
From Cavtat, depending on your interests, you could take a day trip to Mostar, or to Montenegro to see the Bay of Kotor, or you could take a boat trip around the Croatian islands. Lots of options. You could easily spend 5 great nights here. Then fly home from the Dubrovnik airport, which is actually just outside Cavtat.
