That is exactly what several of us were trying to explain . . . I know it is not always easy to arrange an itinerary to do this. Sometimes it takes serious re-thinking of the order of travel. But it is by far the best option. To leave things in a car for hours while off hiking or sightseeing is just asking for trouble.

If one gets away with it - great. But to tell folks it is just fine/hunky dory to leave luggage/valuable in an unattended car is not helpful.

Sure, but it is not always possible, and when someone asks the question on this forum, it is often too late to change the travel plans. The strange thing is that the OP wrote about day trips, so it may be that the luggage issue is really moot: "WE will visit Plitvice NP and Mostar for day trips"

Then why did they ask? It's always possible to change these sorts of plans. They may not want to but to say it isn't possible . . . ? It's a question of logistics & priorities, that's all. One cannot say what's important for someone else but, again, they asked and we gave our experienced assessments. I will say as an upside, if the itinerary dictates that they MUST stop with luggage in the car, what an added thrill when it's still there when they return!

More a sense of relief.

Maybe depends on how good their stuff is?

On that note, I almost always check my main bag when I fly so my rule is not to put anything in that bag I'm not prepared to lose. Not that I wouldn't be mightily annoyed but not the end of the world (or the trip).

Thanks to everyone for the replies. After reading, we have decided to spend one night in each Mostar and Plitvice to avoid the problem entirely.

Stay well

Good decision ;)