Croatia in October- weather, rental car and islands?

I visited Croatia in early October 2009. The weather turns colder at some point during the month (or perhaps early November - who knows these days?). I had great weather in Dubrovnik but the day I left for Korcula, it turned cold and rainy, and I was told the Buza Bar in Dubrovnik closed for the year as soon as that happened. Obviously the weather differs from year to year. Could still be great while you are there, could be cold and rainy already. You're always playing the odds with the weather. If I were going back in the fall, I'd probably go end of September vs. early October, personally. Some things start to end for the season by October (fewer excursions, shorter hours for places). On the upside, mid-October means fewer crowds.

If I were renting a car in Dubrovnik now (I agree, you don't want/need one while there), I'd simply look for the best rates for a "downtown" location on rental sites like EconomyCarRentals (which I used once), Kemwel, Auto Europe, etc. Then, see where their rental office is vs. where you are planning to stay. Then it's just a matter of getting to the car rental office: by bus, taxi, Uber, whatever. Sometimes they will pick you up or even deliver the car to you wherever you are.

Regarding islands: I only visited Korcula, and I did it without a car. That would be too long of a day trip for me personally, but it wouldn't be so bad to spend a few nights there on the way north. However, the island towns do start slowing down as the season ends in October. Korcula was dead at night the nights I stayed there, but restaurants were open.

I was there a couple of years ago and the weather broke 23 September, the day before we left. The temperature dropped to c 20 degrees and it rained. It had been sunny and 28 degrees prior to this. Places on the islands were getting ready to close at the end of September for the winter.

I would decide when you get there as to whether the weather or the sea conditions are suitable for an island day trip. The winter ferry timetable starts c 7 October, so you will find fewer ferries for your stay, so check Krilo and Jadrolinija timetables carefully.

Northern Croatia will be cooler than Dubrovnik on average. At this time of year, it may make sense to start north and head south, if possible.

I spent time in the Balkans in 2015, ending my trip with about a week in Zagreb before flying home on October 12. The weather began to change in Bulgaria during the 3rd week of September. It wasn't cold, but many days included some rain. There was also rain in Montegro at the end of the month; on one occasion I was basically stuck in a restaurant for an extra 30 minutes. I was rained on my one evening in Zadar (around October 5) and on multiple days during my time in Zagreb. By the day of the deluge (October 10, I think) it was cool enough that it was unpleasant to wander around in sodden slacks. The temperature would have been OK if not for the rain.