I strongly recommend a visit to Mostar. You can rent a car to do that, but depending on how many of you there are, it might be economical and much less stressful to hire a private guide. Same with Kotor. The Bay of Kotor is more about scenic beauty; Mostar is about the history and the culture and the people (as well as being a lovely city).
If you do hire a private guide, do your research in advance and be specific about what you want to see and where you want to stop en route. Otherwise, you might get a somewhat "canned" tour that is not customized to your interests.
If you rent a car, you can go wherever you want, obviously, but you will have to deal with parking and traffic.
I wouldn't recommend a group tour because I don't like that sort of thing, but if that works for you, it's the cheapest option.
If you'd like some specifics, feel free to send me a private message and I'll be happy to share more details.