I haven't been to Montenegro, but if you're thinking of travelling around Croatia, the drive up the coast to Split is amazing. The road is small, windy, but scenic and there are a lot of little stops along the way. You drive through a small part of Bosnia as well. With that being said, Split is boring and there isn't much to see there. I stayed at Le Meridien Split, amazing hotel, amazing views. The rest of it is underwhelming. The old city is nice to see, but you can do it in an afternoon. I'd highly recommend doing a road trip from Dubrovnik to Split to Zagraeb. Do the whole country.
Off the beaten path, towards the interior of the country, there isn't much to see aside from rolling hills and arid land. The farther north you get, the interior becomes a completely different place. There a National Park somewhere between Split and Zagareb, it's a must see. Wonderful place!
Montenegro is on my list for sure, but can't help you out yet!