We did a split Montenegro/Croatia two week holiday in 2017.
Kotor Bay is very touristy, especially when the big cruise liners pull in. The Kotor Serpentine up the side of the Black Mountain from which Montenegro takes it name is fun in a hire car, just watch out for mad Germans in huge motor homes coming the other way on the 27 hairpin bends.
Plenty of old castles to be visited, including the old fort at Herceg Novi. It is all a bit dull and rundown away from the tourist areas.
Be aware that if you intend to cross the Montenegri/Croatian border in a hire car, you need a special permit from the hire company.
Cavat in Croatia is very Balkans On Sea, but it does have some nice villas and a very posh part. Best part is the water taxis running to Dubrovnik and back on the hour as getting parked there is a PITA and expensive.