なにかな is a word to tell someone.  
なんだろう is a word to tell yourself.

When you are talked to by someone, you should say なにかな to him or her and you should say なんだろう to yourself.

なにかな means “what do you want to say?” “what is that?” or just “what?”  
なんだろう means “ I wonder what” or just “what?”  
In some Japanese TV programs, a comedian has to answer  what is in a box (It may be a snake or other animals.)  
He says なんだろう. He is just saying “I wonder what’s in the box.” not particularly paying attention to people.
He also says なにかな. He is telling people there “I’m wondering what’s in the box.”