@Diegocorry
"Pupunta ako sa Maynila balang araw" and "Balang araw pupunta ako sa Maynila" both mean essentially the same thing and both are also grammatically correct. Just like their English translations, the first one gives more importance to the act of going while the other, to the time.
If we will follow the subject-predicate structure of "I am going to Manila someday" it becomes “Ako ay pupunta sa Maynila/Manila balang araw”. Using the verb to begin that sentence, it transforms to “Pupunta ako sa Maynila balang araw”, which is exactly your first sentence.
The sentence of BoraMac, “Pupunta AKO NG balang araw SA Maynila”, is incorrect because of the “ng” (of) that he added to it. It translates to “I am going to Manila OF someday”. Without that “ng”, “Pupunta ako balang araw sa Maynila” would also be correct except that, of the 3 versions of the same idea, we’d most likely be using more the two that you gave.