I've been to Hvar in the summer and it was crazy busy. I imagine in October it would be fun to cycle the narrow twisty roads around the island.
Use Google street view... there are zero shoulders on most roads, but I imagine the locals are pretty decent to cyclists.
I've heard the coastal highway is surprisingly good riding and even into Bosnia.
I think you have to get to Makarska and climb Sveti Jure. Huge bucket list climb for me.

We're heading to Croatia at the end of Sept/early Oct. My bf isn't into real road riding, but we'll probably rent bikes one or two days and ride around some of the historic towns. We're going to start in the north in Pula, then to Motovun for the truffle fest, and work our way south into Plitvice Lakes National Park, then to the island of Vis and finishing our trip with three days in Dubrovnik.

While I absolutely love my hometown, and cannot recommend it enough as a vacation spot, Split itself isn't the greatest place to do road cycling if you are staying anywhere within the city limits. If you are ok with loops, you can get a pretty decent ride doing loops of Marjan, and there is a nice little climb to get to the top of the hill. Getting off the peninsula can be a little dicey, as the main roads are effectively highways, and you have to use very much back-roads to get out. Once you are out though, there is some decent riding to be had. The coast road is pretty nice, but I would suggest heading south from Split, as north can be pretty busy until you clear Trogir. Heading inland is also an option, and there are some really nice climbs that way. There is also a little pseudo-peninsula between Trogir and Sibenik, west of the new highway, that has some really nice riding.

I would very much suggest Korcula, my mothers family is from the island, and it is a beautiful place. The roads are fairly quiet, there's a nice mix of flats and some shorter climbs, and the roads are in decent shape. Weather in October should be pretty good, albeit maybe a little chilly, but nothing atrocious, in fact if you're from a colder climate I'd wager there's a chance you could even hop in the sea and not freeze to death. Locals are pretty friendly, and there's enough tractors on the road that they're used to slow moving vehicles. The wine and food are incredible. The city of Korcula is very nice, but also explore the other towns. I personally love Prigradica, but you could probably hit all the "big" villages with one big day in the saddle.

I've never personally ridden on Hvar, but have heard it is pretty good, and I think in the offseason you should have no issues with it being too busy. Same for Dubrovnik.

That sounds like a fun trip. I have seen all those places. You won't be disappointed. Be advised that the weather breaks pretty reliably about third week of September right around calendar fall start. It might rain. A lot.

Pula is lovely. Walk that loop through the old town. On your way south stop at a small town Opatija not far from Pula. It is amazingly beautiful.

Have you seen a line of 1000 tourists walking the trails wearing raincoats, at Niagara? That's what Plitvice looks like. Come early just as the park opens and it will be empty.

Do stop by Split and see the Roman palace.

Dubrovnik... well you'll see. :)

So. Fun anecdote for you. We got lost in the old town in Split, on roads barely wide for a car, in a BMW SUV. It was the only car the rental company had that could fit our bike box and luggage. Car was fine until we got to near our hotel a few streets from Diocletian palace. It's funny now. I didn't think it was then. :D

hahah. I had a similar thing happen in Sicily. We ended up with a little Fiat Panda and my bf and I were laughing at how tiny it was (yes, we're American and drive a pick up truck), then I started driving around Palermo trying to find our hotel and was really glad that we had a tiny little car to fit through the alley ways. I saw one too many BMWs and Alfa Romeo's having to back out of one way roads to avoid getting stuck!