Now have an extra two nights in Croatia...where to spend it? What to do?

What about Šibenik? We haven’t been yet, but it is on my list for next year. They have many tourists but they don’t seem to be Americans.

Are the extra 2 nights before Split or right after Split? If before Split, I recommend spending 2 nights in Hvar Town on the island of Hvar. You could take the ferry from Korcula to Hvar. We visited as a daytrip from Split, and we would have loved to spend a night or 2 on Hvar. Hvar Town is beautiful! There is a gorgeous uphill walk to the fortress with beautiful views of the harbor and the Adriatic. Then a stunning coastal walk (flat) that starts in Hvar Town. Hvar Town has a reputation of being a party town but that is during the summer months. We were there in mid to late September, and there were tourists, of course, but it really wasn't that bad. No drunk people walking around.

If the 2 nights are after Split and before Plitvice, then Zadar would be a good choice. We haven't been there but our daughter and son-in-law were in Zadar, and they loved it.

Just to clarify, the extra two nights are after Split. Or should we spend extra nights in Split and daytrip from there? If so, what are the recommendations for doing this?

I think Kmkwoo’s suggestion of two nights on Hvar are time you’d really enjoy. September is perfect for the Dalmatian coast and Hvar has a castle above the town that makes for a nice easy hike. Another beautiful hike IIs along Hvar Town’s boardwalk toward the Amfora Hotel and beyond it. There are cafes, some oceanfront mansions and beautiful coves along the boardwalk. One of the oldest continuously-inhabited towns in the world, STARI GRAD, can be reached by local bus leaving from near the grocery store by Hvar Town’s cathedral. Hvar is a couple hours by Krilo catamaran from Korcula, and about one hour by Krilo catamaran from Split.

Kenko, the coastal walk that I mention is the same one that you suggest, towards the Amfora Hotel. We thought it was stunning!

HH05, if you spend the additional 2 nights in Split, you can take a daytrip from Split to Hvar Town. That is what we did. And then take a day trip to Trogir. We spent about 3/4 of a day in Trogir. We stayed 4 nights in Split and thought it was perfect. FYI, we hired Maja Benzon, who Rick recommends, for a private 2-hour guided tour of Diocletian's Palace and Old Town Split.

If you can re-arrange your hotel accommodations, I think spending 2 nights in Hvar Town BEFORE Split would be ideal.

Edited to add that if you spend 4 nights in Split, and if the catamaran schedules work out, you could stop in Hvar Town for the day on your way from Korcula to Split, and then take the catamaran to Split in the evening on that same day. But my first preference would be to spend 2 nights in Hvar Town.

https://youtu.be/Fhmzu7p9rus

@Kmkwoo: It was really relaxing in Hvar staying 4 nights at the Villa Hosta Apartments, which actually is one of the first guest houses begun in Hvar by the grandfather of today’s hostess, Stefania, along with her husband Roberto. Wonderful and gracious hosts, they have four apartments in their large compound overlooking Amfora beach. The boardwalk was right below our 600 sq ft patio and beyond it was the beach. After watching so many people walk along the boardwalk below our private patio discreetly shielded by olive trees, I finally took a stroll to see what the attraction was further down the boardwalk. I was amazed at the beauty of the turquoise coves which had small beaches. There was even a “dog beach” on one of the coves reserved for our fine four-legged friends. There are a number of beachfront mansions on the boardwalk ( which hints at the desirability of the location) that have been repurposed into hotels and restaurants over the years.
I couldn’t understand why everyone says Hvar is a “party town” after staying there in September. I think you nailed it— the party and the crowds happen in July and August.
We stayed in Split inside the walls of Diocletian’s Palace, named after the only Roman emperor to retire ( almost all the others had unfortunate fates). I’m a bit of a history enthusiast, so I booked a room at “Golden Gate Rooms”, which had a room where it’s northern wall actually is a portion of the Golden Gate entrance to Diocletian’s Palace! Historians believe it was the location of the barracks where Roman centurions slept while off-duty protecting Diocletian at his retirement palace.
Wherever you stay in Split- be sure you stay in Diocletian’s Palace which is the center of the action in Split and a unique experience to be had nowhere else. The thickness of the walls of the stone buildings means that you sleep well wherever you stay.
Happy New Year!

Sibenik and Zadar are both worth visiting. There is a fabulous turtle fountain in Sibenik that is amazing. It's a small town, but fun to explore. The Franciscan garden is worth the trip. Zadar is also quirky. It has the sea organ. It also has the best sunset in Croatia, according to someone I forget.

Sibenik is a neat town, yes.

On my recent trip back to Croatia in October, I spent a few days island hopping from Split to different islands like Hvar, Brac, and Vis. They were all nice. Hvar wasn't really my cup of tea. Brac was nice but pretty ordinary. But for some reason, I LOVED Vis! It just seemed more...organic somehow? Hard to explain. In any case, I planned to do only a day trip but wound up coming back for two nights after.

Island hopping was fun. The ferries are cheap and service is good. I enjoyed the ferry rides themselves also!