Yep. It's absolutely correct translation.

What I don't like about your choice is that... With "когда-нибудь" topicalized (moved in front): "я обещаю, когда-нибудь/однажды...." it is emphasized, it is a separate intonational unit, it invites the listener to 'imagine' that moment before the rest is said. Where you used it "...мы когда-нибудь..." it is more casual, it merely describes the rest. It's more cursory. And I'm afraid this 'casuality' interacts with 'randomness' of -нибудь. Not too assuring, given that it includes very distant time too. But may be it's a mere hallucination. When you analyze foregners' texts, you read things into it, which you'd never see there when saying it yourself:)

Sam, perhaps for more sensual perception etymology can help. "что-нибудь" literally means "what ever [it] be". 'Ни' is used similarly to 'ever' (being very diffent in its original meaning), it has a shade of 'randomness', 'anyness'. E.g. 'ни' in "никто не...." can be interpreted as "not any single one (of people)" 'будь' is 'be' (imperative, may be originally addressing 'it':)) For your exmaple you have 3 options: как-нибудь, когда-нибудь and однажды. ("Как-" here doesn't mean 'way of action', it refers to 'some situation') как-нибудь is very casual. Ничего, если я к тебе как-нибудь зайду на чашку чая? Когда буду проходить мимо? Ничего если я её занесу как-нибудь на днях? (about an object I must pass to you) "would you mind if..." It has a shade of 'at appropriate occasion'. Being casual it makes it sounds light-minded. It makes the promice less solemn, but also it means that it is more immediate concern: part of foressable future, part of the future for which you have plans. Which sounds good (if you mean this). однажды is more bookish. It can refer to any point in abstract time. May be distant. Bookishness 'strenghtens' the promice a bit... but когда-нибудь is fine too. когда-нибудь also refers to any point in time, also abstract and may mean very distant future.

Yeah, sounds perfectly natural to me.. There might be some variations in the word order.. e.g. (Я) обещаю, что когда-нибудь мы поедем в Испанию вместе. But all in all it's OK)))