Rode down through Croatia and Montenegro a couple of summers ago. Both are beautiful, especially if you still to the coastline. Montenegro is more rustic and you can drive up into the "black mountains" after which it was named from the Bay of Kotor. Croatia is slightly more touristy but still amazing. Don't miss Split & Dubrovnik.
Rijeka is not so great. Zadar is nice - like a quieter, less touristy version of Dubrovnik and Split mixed together.
There are also all the Croatian penninsulars and islands that are so beautiful, with such clear, blue water it's unbelievable.
Unfortunately I was there in August so I can't vouch for the weather, but I can imagine it'll be pretty pleasant still.
Enjoy!