I am sure most people here are familiar online courses / MOOCs. There are a couple sites that aggregate available courses. One that might be usefully for home educators is https://www.coursebuffet.com It organizes MOOCs like a university courses catalog so one has a general idea if a course is a 101 type courses or an upper level course. This makes it easier to compare courses directly and plan learning.
There are lot of intro course that higher school age students could take.