It would be good if duo showed the contextual meaning of the words in addition to the literal meaning of each word. Hard to guess this one on your first time around.
I think 一下 is pretty colloquial, 你进来一下。Nǐ jìn lai yí xià. (Come in for a minute.) Is another.
For people wondering: 一下 softens the tone and I have been told it is also quite polite
请看一看 is also accepted.