1) Theory - Work on their knowledge of reading music. This will be different for every student but you will know fairly quickly what their reading level is. From there you can choose what to focus on.
2) Listening - Pull up some youtube videos of people performing and listen to them with your student using share screen/share computer sound. Ask them questions about what they notice and what they heard.
3) Perform - Every student LOVES to hear their teacher perform. Choose a piece that shows something they have been working on…coordination, expression…etc. Ask them what they noticed in your performance.
4) Composing - This can come in various forms. Compose using only finger numbers, scale degrees, rhythmic subdivisions…etc. Just make sure it is written down in some way so you can revisit it and expand on it.
5) Improv - Find something they is level appropriate and show them how to start simply. Ex: Pick a scale, have them choose 1 note to improv with, expand to 2 notes…etc. There are tons of backing tracks to improvise along to on youtube if you want to play along with something.
6) Change the Order - If you normally start with scales, put them last…etc. This simple switch can grab and hold your student’s attention since they will notice something is different. They will be more on their toes waiting to see what else might be different.
7) Online Games - There are tons of free online games about music. You can do these together via share screen. I would suggest starting on www.musictheory.net
8) Invite a Friend - Do you have a significant other or a roommate who could join you for the last 5 mins of your lesson? They don’t need to be musically inclined…just willing to listen. This gives the student a mini performance experience.
9) Sight Reading - In online lessons it is especially likely to overlook certain skills like sight reading. Through sites like https://opensheetmusiceducation.org/generate-sight-reading-practice/ you can generate endless sight reading material catered to your student’s skills.
10) Share Music - Nothing will give you more insight into your student’s tastes and musical ambitions than finding out what music they care about. If you share the music that you are passionate about and tell them why you care about it. They are more likely to offer up the same in exchange.