We drove to Kotor in Montenegro in a day trip from Dubrovnik. Kotor is beautiful, it did take a while to get there (probably felt longer because our map finished at just outside Croatia so wasn't really sure how much further we had to go). I think it took about 3 hours but the journey is part of the experience so I didn't mind that. There is a lovely parking area viewpoint on the way round Kotor Bay that looks across to Persat with a hotel on an island. The mountains come right down to the sea, it really is a beautiful area.
Have you thought about Plitvice and Krka national parks while in Croatia? They are amazing.
We didn't go to Mostar but there was a day trip available from our hotel in Dubrovnik. We flew to Spilt but didn't stay there so don't know what tours were offered from there. Have a look at some of my photos here
http://www.flickr.com/photos/shadow17horse/