Work trip early June - 1 full free day ideas?

I would look into taking the Ferry from Dubrovnik to Split. It looks like there are at least trips 5 times a day. It would be more fun than having to drive in crazy traffic with no parking near the sites. I haven't been there for years, but friends of mine have enjoyed going by water to visit the Islands. I've heard that Split has become more of a shopping destination.

A visit to Lokrum National Park on Lokrum island would be much easier. ( Lokrum is a 15-minute ferry ride from Dubrovnik’s Old Town pier) and much more relaxing than spending most of the day getting to and from Split. Lokrum is a forested island with an 11th-century Benedictine monastery, a fort that Napolean built, and beautiful beaches.

Split was the least favorite destinations during our time in Croatia. We really enjoyed our day trip to Kotor, and you could drive there in half the amount of time. That may be worth considering.

I don't think Split is worth the effort for one day. For a day trip, Mostar would be better than Kotor because it's different. When are you going? If the weather is nice, you could tour the nearby islands or spend a day in Cavtat.

TravelBug79 Split is my least favorite place in Croatia too and I went twice to make sure. Kotor is pretty nice I guess. In general I love Montenegro but after Dubrovnik, Kotor is sort of a less facinating small brother. Here is my two cents on what to do when you get have too much time in Dubrovnik: https://community.ricksteves.com/travel-forum/croatia/daytrip-to-montenegro-from-dubrovnik.

If you are into hiking then Lokrum Island looks interesting. Four hours of walking will apparently get you to most of what folks go to see. So, with and hour each way, its a full day.

Thank you for valuable feedback - I appreciate your input!