I would first figure out what kind of learner/studier they are. If they are audio/visual people, I would send them youtube links of people showcasing skills (like Crash Course or Khan Academy videos). If they are tactile I would send them online websites like practice exams.

I would create content with my college aged friends. I think people should never stop learning so this fun can go onto older kids. Some of my friends still act like toddlers and I'm actively teaching them STEM things like why green and red peppers are the same thing and how to transfer files on their computer. So I think working with older kids would be fun and spread to older kids as well and how we can all have fun while learning.