I'd repair our education system or replace it with something that works.

I'd visit the near future, close enough that someone might want to talk to Larry Niven and can figure out the language distant enough to get me decent medical techniques and a ticket to the Moon.

In hindsight it may even seem inevitable that a socialist society will starve when it runs out of capitalists.

Building one space station for everyone was and is insane: we should have built a dozen.