わたしは今日、日本にかえります。
This is actually the most natural (or perhaps I should say neutral) word order and topic marking here. It’s common in speech to place a slight pause after 今日 (represented by a comma here).
The reason for this is because it’s most natural for the subject of the predicate to become the sentence topic in Japanese. This is what happens by default, and as a result it doesn’t add any layer of extra nuance or markedness.
You can mark other things in the sentence as a topic, such as the indirect object (日本 here), time adjuncts (今日, 今{いま}, 明日{あした}, etc.), objects (no example available from this sentence), but this goes against the default, which means there is some reason for doing so in terms of how you are trying to structure the information.
The basic function of a non-default usage of は is to pick something out of the discourse and make a comment specifically about it (and not other things).
So if you said 「今日は日本にかえります」, then that would only be natural if someone had either already brought up “today” (e.g., by asking you what you’re doing today) or you are for some reason bringing it up yourself (e.g., to contrast with what you’re doing tomorrow, or to give it a sense of ‘at long last’ by contrasting it with all the days up to this point).
If you said 「日本には今日かえります」 without someone else having already brought up 日本, it’d be a little odd, because you can normally only かえる to one place anyways, so there is no need to emphasize that you are not かえるing to some other place also. But following a question of 「日本にはいつかえるのですか？」 (which itself would only be a natural question if there is some reason to believe they are already determined to go back to Japan), it would work (though in almost all cases you would just say 「今日です」 or 「今日かえります」 to be more succinct).
