Our tour in Croatia was amazing and so was Split.
It's not as
unique as, say, Dubrovnik, which almost seems stuck in medieval times.
While Split has a lot of history (with Diocletian's Palace), it is a modern city and - unlike Dubrovnik - it's not overrun by tourists, although you'll still find a ton of them.
Split's main attraction is the remains of the palace where Diocletian spent his post-emperor days. Apparently, this is (one of) the best preserved Roman palace(s) in the world, especially the basement halls. Nowadays, part of the basement is filled with market stalls selling all sorts of souvenirs, but for a small fee of 40kn (around $6) you'll get access to the rest of the excavated underground remains.
Much less remains of the ground floor, where it is now hard to distinguish remains of the palace from the rest of the city. Over hundreds of years, the city has grown on these remains and the two are interwoven.
Central to the palace is the Peristyle: Diocletian's ceremonial court, which includes a sphinx he'd brought in from Egypt. Have a drink on the steps around the Peristyle and you'll feel like you're back in Roman times. In the Middle Ages, Diocletian's mausoleum was turned into the St. Duje Cathedral & after the addition of the bell tower (climb it, the view is amazing!) this square became a religious center.
Bust most of Diocletian's Palace's upper floors hasn't survived and for the most part it's turned into homes and shops. Diocletian's Palace makes up about half of the very busy and touristy historic center. On the south side is the Riva, the waterfront promenade. It's very modern & contrasts beautifully (in my opinion) with the old buildings just a few meters away.
I really would've wanted to go visit the Blue Grotto, a cave where the sunlight reflects in such a way that it radiates blue hues. It's supposed to be wonderful, but it's a few hours away by boat. We didn't really want to be on a boat all day so we didn't go, but it's still on my list should I ever go back!
We did end up going to an island much closer to Split: Brač - the biggest of the Dalmatian Islands. We took the ferry to Supetar, then took the bus to Bol, a town on the other side of the island. There really is no particular reason to come to Bol other than Zlatni Rat, one of the world's prettiest beaches with white pebbles, perfect turquoise waters, a pine tree grove bordering the beach and a backdrop of the Brač hills. I'm not much of a beach person, but this was perfect.