NOTE: The important blue button mentioned prominently in the video here no longer exists. However, the rest of the information in the video is correct.
Google Assignments is an "external tool" for Canvas that enables some really great Google Drive file management features for you and your students. For instance, it can create copies of files that you prepare for your students - including adding the students' names to the file name and avoiding the "Untitled document" tsunami. It also automatically handles all the file permission changes that need to happen between the student completing the assignment and submitting it, to your grading and returning it to them.
Check out this video walk through to see how both the teacher and student side of this tool works.
UPDATE: Google Assignments now features a great rubric tool which works not unlike the Canvas rubric tool. Not only can it serve as a reference or guide for your students and your grading, it can give you a super-efficient way to grade according to that guide. See the video at the following link to see a walk-through of how the tool works: https://www.youtube.com/watch?v=OnueoPKr2mY
TIP: You can access all your Google Assignments courses and assignments without going through Canvas at: assignments.google.com/ca
You cannot make an assignment for Canvas from this site, though. That still needs done in Canvas itself.
(NOTE: This also works for students, but it's probably best to keep their experience with Google Assignments anchored in Canvas.)
direct link to GDrive to download video: https://drive.google.com/open?id=11XoCvOKNAdfnfdjYgywuEsfkGJhdk4MR