With 6 to 10 days the choice is Croatia with maybe a side trip up to Mostar. You must see Split, Hvar and Dubrovnik but beyond that there are all sorts of options that really depend on your interests. For instance Nature, then Plitvice and Kravica falls. Food and drink there are a number of good wineries some with accommodation and an olive plantation can br a very interesting experience. For sport kayaking, sea and river fishing, hiking. Cultural might call for an extra day in Split and Dubrovnik and a trip to the Blagaj Tekija monastery. Village tourism ... ???
How late in October, cause October is getting marginal for coastal fun and the cities become more important. If you agree then Split, Dubrovnik, Mostar, Sarajevo, home works out to 10 days. Its probably what I would do in late October. There are a good number of options for sight seeing stops inbetween the cities. But im clicking on a phone and too much spelling to look up unless you are interested.