Rental Car Company Recos for Dubrovnik Day Trips

Any reason you want to rent a car and watch the road instead of the views?

Dubrovnik to Mostar is about 2.5 hours if all goes well. Its 140 to 150km so that works out to and average of 55 to 60km per hour (35 to 40mph). Why so slow? I imaging the nature of the road requires a lot of attention. Sunset will be 7pm, might be good to be back to the room by then. Leave at 9am and that gives you 10 hours for the trip. Lets round the round trip up to 7 to 8 hours round trip including time to pickup and return the car and border crossings. That leaves you 2 to 3 hours in Mostar. Doubt it will much cheaper than hiring a transfer and sure it will cost more than taking a tourist shuttle or group tour.

Dubrovnik to Kotor is 2 hours. Figure 6 to 7 round trip with time to pick up and retun the car and border crossings. So you get 3 to 4 hours to divide between Herceg Novi, Perast and Kotor.

For the tours do some google work. For a private transfer (the right thing to do) there are a number of sources. I like letting a lady named Dijana handle everything. Here are some references with contact information:
https://community.ricksteves.com/travel-forum/croatia/great-travel-agent-for-montenegro-and-croatia
https://community.ricksteves.com/travel-forum/montenegro/montenegro-help
https://community.ricksteves.com/travel-forum/czech/prague-budapest-and-dubrovnik
https://community.ricksteves.com/travel-forum/trip-reports/16-nights-in-montenegro

Here are some other suggestions: https://community.ricksteves.com/travel-forum/croatia/daytrip-to-montenegro-from-dubrovnik

I too would highly recommend that you book a tour or transfer service rather than renting a car. My suggestion is that you check out Petar Vlasic, who is an awesome guide. We used his services for three day trips. https://www.dubrovnikrivieratours.com/