in a few days
或者是a few days later都是可以的。
例：
The matter will rectify itself in a few days.
那件事過幾天就會變好。
I am leaving for sydney in a few days' time.
過幾天我要到雪梨去了。
In a few days, you will meet your baby brother.
再過幾天，你就會看到你的弟弟了。
one of these days
總有一天（用於許諾），也許明天，有朝一日
英文解釋是within an indefinite time or at an unspecified future time.
（在一個不確定的時間或在未來的一個不確定的時間）
例：
Good night, old chap. I'll be over again one of these days.
晚安,老弟。我過幾天還會再來的。
" One of these days, " said Fanny, with dismal emphasis, " you'll get into trouble."
「過不了幾天，」範尼難過地強調說，『你就會遇到麻煩的。」
sooner or later
過不了多久，遲早
used to say that you do not know exactly when something will happen, but you are certain that it will happen
來表示你不知道某件事什麼時候會發生，但你肯定它會發生；遲早
●Sooner or later, it will out.
在不久後，它會逃脫出來。
●Sooner or later she's going to realize what a mistake she's made.
她遲早會意識到她犯了一個多麼嚴重的錯誤。