There is an excellent series on the Chronicle on making small changes in teaching. I love this idea of retrieval practice. Rather than testing the students or you just giving them more content in the form of a review actually ask them questions. Make them think about it, write it down, then choose a couple to recap what has previously been learned. You can really make the most of the first five minutes of class.
“Instead of "testing effect," I prefer to use the more technical term, "retrieval practice," because testing is not required to help students practice retrieving material from their memories. Any effort they make to remember course content — without the help of notes or texts — will benefit their learning.”
In an online context this can be done in a simple discussion forum or even a short weekly video conference that is 10-15 minutes long. Students will find value in this as they will learn what content to focus on while also practicing a review in a non-pressure situation.
Source: Small Changes in Teaching: The First 5 Minutes of Class - The Chronicle of Higher Education