Croatia in early October

Have you scoped out the transport options? Rijeka is less of a draw, but I found Opatija quite pleasant.
Looks pretty good, but you might consider just stopping in Sibenik for an afternoon and add that night at the end to account for longer driving and give you a chance to see something of Istria/Kvarner region.
We need to know what you have on your list to evaluate number of days allotments. I have always heard Istria is nice in October, truffle season commencing and such.

I’m not seeing any overnights on the islands, and they’re wonderful. If the ferries are running, I would definitely at least spend a day at Hvar near Split….but check ferry schedules!

I just noticed you said early October, so it may be too early for truffles as I had mentioned.

Is it normal for you to do one night stops? For me 3 in a row would not be fun. J

Agreed on the 1 nights..... our plan was to do a bunch of day trips from Split and then work our way up the coast ending in Venice....

Maybe do Sibenik on the way to the park and add a night in either Rijeka or Opatija?

That's what I would do, unless you determine you can travel just as easily from Rovinj, which I have not been to but gets a lot of love on the forum.

October and Croatia starts to draw the curtains on the tourist season, still should have a range of restaurants open to choose. Water temps drop quite a bit so expect to turn blue if you choose to swim.
Consider using the travel mode of the eons by taking ferries up the coast for you both will enjoy the experience (leave the driving to the ferry captain).
2 nights Dubrovnik, ferry
2 nights nights Hvar, ferry
2 nights Split and then pick up car and drive to Zadar
2 nights Zadar
1 night Plitvice
1 night Rijeka

It is possible for falling weather to begin showing up in early October. I don't know,whether it was statistically unusual, but I had some wet days in Montenegro and Croatia early that month in 2015. However, the worst day was in Zagreb (rain, rain and more rain), and you're mostly sticking to the coast. I mention this just so you can think about indoor attractions that might be of interest if you're umlucky with the weather.

Steven:

Water temps drop quite a bit so expect to turn blue if you choose to swim.

I take it you have never been swimming off the Dalmatian Coast in early October. Even with falling temperatures, the water stays warm for quite a while - they don't drop for a while after the air temperature falls. While it's not like swimming in August, once you get in the water is fine and you can stay in for a while if you want to.

gayleyost1, since you are headed to Venice, let me suggest a detour to Ljubljana, Slovenia for your final night or two. You might try this:

Dubrovnik (2 or 3 nights)
Split (3 or 4 nights)
Sibenik (1 night)
Plitvice (1 night)
Drop car in Rijeka, take the train to Ljubljana
Ljubljana (2 nights)
Bus or GoOpti shuttle to Venice

I have been to Croatia in early October a few times - just last October most recently. The weather gets more iffy the later you go into October. Rain storms and cooler temperatures become more common, but you can still get some great weather. Last year, I arrived in Split just after a few days of storms (which I recalled from my previous October trip there) and had five days island hopping without a drop of rain and plenty of sun. The nice thing about Croatia in early October is that things start to slow down so it's much less crowded. (But I would still expect Dubrovnik, Split, and Plitvice to be fairly busy.)

If you have rain, you might skip Plitvice. It's an amazing park, but it's not going to be much fun tramping through the mud and through puddles in the rain. Have a rain plan. In October, last-minute accommodation should be fairly easy to find if you want to change things up on the fly, if you have a mobile device for making reservations and changes. At least there are some museums in Split. You might go up to Ljubljana a day early - really lovely little city, no world-class museums, but at least there are some. There is just more to do there in the rain than in the coastal towns. Or drive up to Zagreb and drop the car there and spend a night or two there, more rain activities there than in the parks and on the coast. Take the train to Ljubljana from Zagreb.

Near Sibenik is Krka National Park. If you get rain there, at least you can take the boat out to the amazing Skradinski Buk waterfall - larger than anything at Plitvice. You don't really have to hike much to see it and enjoy it (in nice weather you can do more). But at Plitvice, you really need to hike to enjoy it - it's great but you have to hike to see all of the dozens of medium and small waterfalls and few larger ones. Just not going to be as fun slogging in the rain.

Why Rijeka or Opatija? Opatija, maybe. Rijeka, an ugly port town? I mean you can drive just a bit further and hit the paradise that is the west coast of Istria…

Fazana, Rovinj, Porec and the charming Vrsar. No competition.

We are spending 2 weeks mid Sept - Oct 2

4 nights Split - visiting Krka as a day trip
3 nights Korcula
4 nights Dubrovnik renting car on last full day for day trip to Montenegro
2 nights Mostar as we still have car then returning to Split to drop car and fly out

We tried to limit our time getting to and from places. We have found that we prefer a 3-4 night stay and doing day trips out of that location.

We were in Dubrovnik in 2015 Nov 2-5. It was cool, and sometimes rainy. There were no crowds. We stayed high above the city (425 steps to the Strada). We enjoyed the less-touristy time there.

Actually have been to Croatia in October and only write about actual travel experiences. Took a seven day small ship cruise and we did turn blue upon entering the water. It takes hearty soul/body to immerse into the chilled waters of Croatia in October, but having done so leave s you with the ability to say "Been there, done that".

I have been swimming several times off the Dalmatian coast in October and certainly did not turn blue. Last year I went swimming at Vis, and on a past October trip I have been swimming by Dubrovnik. Once I got in the water was fine and I was able to stay in a while. The BnB owner in Dubrovnik went swimming every night and once night I went with him. I suppose water temperatures vary depending on exactly where you are or how warm it was that year, but clearly it's possible to go swimming in Croatia in October even if it's not the ideal time to go.

My family is planning to go next October, base in Rovinj and take a few day trips. The plan was to take the ferry to Venice for our next city on the itinerary but it looks like the ferry doesn't run much, if at all in October. Looking at the Venezia Lines webpage, it does state they have limited service in October between Rovinj and Venice but there were no dates indicated for this October when I looked to get an idea of schedule and cost for planning purposes. Does anyone have experience with the ferry in October? Maybe another line? We are still flexible at this point to switch the itinerary to go from Venice to Rovinj instead.

The other option is to drive. My husband isn't to keen on the idea but I'd love some input on that. Is the drive difficult, rental car pick up/drop off a hassle? Not meaning to hijack the OP's post but maybe gayleyost1 will get some benefit as well!

https://www.visit-croatia.co.uk/travelling-around-croatia/ferries-croatia/

Another option would be to take the bus from Rovinj to Venice...or a bus to Trieste and from there take the train to Venice (the bus station is adjacent to the train station in Trieste). I see there are some direct buses in October that take about two hours to Trieste, just over four hours on to Venice, and the train from Trieste is about two hours on to Venice. This will take longer and seem like less fun than a catamaran ride, but in my experience the catamaran rides can be pretty bumpy anyway.

There's also a Slovenian shuttle service called GoOpti that may be able to get you there a little more conveniently (but more expensively) than a bus/train.