If I had time ...
If I knew I was going to be absent, and had time to prepare ... I loved using online apps that capture students' work.
ClassKick worked especially well for me. I loved setting up a lesson in it ... I could record instructions, insert a video, give explanations as well as capture students' work. I used ClassKick while I was out at a professional development training. Whenever I had a few minutes, I logged in and gave feedback to students - even while I was absent from class! Students were surprised and they liked it!
I also planned lessons in Desmos when I knew in advance that I would have a substitute. I loved being able to capture students' work - certainly helped to hold students accountable in my absence.
Sometimes, I set up a series of videos on our class website and a set of practice problems to go with them. That way students were still learning and working. The videos I used most often were published by PatrickJMT. I usually left a note with subs that it was OK by me if students worked together on that practice.
On emergency absence days ... when there was no time ...
Sometimes, even on emergency days, students would complete the planned lesson. Our PLC planned units in advance, and most often my copies were ready. So if the lesson wasn't totally new to students, they could complete at least part of the planned lesson. A PLC colleague might also suggest a video or a review assignment - we tried to take care of one another on those days.
I also used Desmos on emergency absence days. I'd email instructions to my PLC lead or a colleague next door. These are some of the Desmos activities I used for sub days:
Match My Line
Match My Parabola
I might use other pre-made Desmos activities on emergency days - depending on how much time I had to locate one that might work well. I like the bundled activities!
I also kept a few puzzles on hand for subs. Algebra with Pizzazz is an old book. I was surprised to find a copy of the book online, as well as the Pre-Algebra version. Another one I found is Making Practice Fun. Each marking period I would print off a class set of 4 - 6 puzzles and put them in a folder with my sub folder. I would choose skills that I knew students needed to practice.
Speaking of my sub folder ... our school had a set of requirements for sub folders. But they were pretty standard. I kept my schedule, duty assignments, attendance sheets, seating charts, and a page with generic instructions. For example, I always said that unless I left a test, that students could work together. I allowed students to have their phones (unless there was a test) and said they could listen to their music. Our school had a standard form for substitute feedback. It asked for names of students who were absent, for what went well, and any issues that might have arisen.
Bottom line - I tried not to be too anxious about sub days. I tried to provide students a learning activity but most often it was not graded. I was blessed to not be out very often. And I had a great PLC. I'm sure I would have felt differently if I had to miss several days but even at that I know our PLC would have helped!