Xiaobaitu, Pjiang, plesae help about travelling to Canada, Thank

I am now in AOS status (NIW I-140 approved, I-485 pending, using EAD to work, no AP). I will attend a meeting in Canada in May. I know I should apply a Canada visa. My question is how I can go back to US after the meeting (less than one week)? What document do I need to reentry US?

Thank you!