If you'd like to leave a review before you return the book, you can do so by clicking on the star rating for the book on your dashboard.
You can also leave a written review as well as, or instead of, a star rating. Click on "Leave a review" and fill in the box.
The reviewer's name is displayed by clicking on the drop-down menu next to it and you can choose to display your full name, initials or just anonymously as a Calibre Member.