If you read the other threads you will see that private and public families supplement similar things: college advisors, Sat prep, instrument lessons, tutors when needed, summer programs (although I’m not sure that’s really school supplementation), theater, travel or specialized sports, independent athletic coaching. It depends on what your kid needs and what the school offers. You can find current conversations right now stating 1/2 the kids at GDS hire college counselors and J-R students do as well. Best advice, get a job in that and pay for whatever your kid wants.

Outschool. Particularly FLEX, because students can access the content and assignments on their own schedule.

Study abroad in HS
Club sports
Private college consultant

Tutors.

Summer programs at universities

Instruments

For us it is SAT/ACT Prep and private college admissions/application consultant.

Will also pay for any outside AP test prep if kid asks for it, though that is primarily online sources of study and test prep materials. Our kid's HS actually does a pretty good job getting kids prepared for the AP tests in most classes.

I don't think people really do that in high school. Folks use tutors to get ahead and more in depth understanding in math and to become better writers. It's really next to impossible to do in high school with all the extracurriculars.