For nature-lovers, Plitvice National Park is not to be missed, assuming you are not incredibly unlucky with the weather (seeing the park in a downpour wouldn't be pleasant). It's important to know that you need to spend the night before your park visit near the park so you can get an early start the next day. The park gets a lot of day-trippers. I'm sure it's not as bad in October as it is in mid-summer, but I think you'll still benefit greatly by getting out on the walkways when the park first opens. They were unbelievably, miserably packed during my summer 2015 visit. There was also the small matter of waiting an hour in the ticket line.