I'd have probably done Earth science or geography in addition to my current subjects. My future ATAR is still ass kissing me after I changed from standard to advanced english earlier this year
I would go back to 1999 and would warn the world about events to prepare for and try to stop (i.e. 9/11, 2004 Boxing Day Tsunami, various earthquakes etc).
I WISH I wasn't doing my LOTE subject!! I haven't done it since year 8 but I could speak the language due to my parents. Do LOTE, they said, I'll be easy, they said. I really should've thought that one through as I couldn't read or write at the beginning of course. I'm just really, really stupid. Should've done drama instead!