I strongly recommend you to go to:
Belgrade - because there are so many interesting things to see (monuments, nature, buildings, museums, etc) and also because Serbs are very special. The reason is for that is simple: their origin is here in Serbia, but their heart belongs to the whole world. That's why I'm pretty sure you're going to spend wonderful time here.
Greece (Athene, Paralia, Krf, Katherini, Solun, ...) because Greeks are so similar to us in the sense they're also cheerful and joyful, and they're so nice and kind, just like us. Also, they have wonderful beaches. If you go there, I strongly recommend you to eat their traditional food Souvlaki.
I visited tones of times Montenegro and I really love that country. They have beautiful beaches, but mountians too (like for example: Durmitor) and much more. I'd recommend you maybe Podgorica (capital city), Budva, Herceg Novi, ...