Look, causality or quantum physics aside:

On 9-11 instead of being in college and sleeping in I would call in a bomb threat to the WTC

I would call the police to stop various school shootings

I would put money in the stocks to profit from various recessions

I would tell mom and dad to invest in Apple stocks back in 1985 - the worst thing that can happen is that by doing that, Apple doesn't come up with the idea of the itunes store and ipods.

Like milf said, I would leverage my 31 years of experience into 20-25 more years of the same. Granted I'm not as smart as people who are well versed in college level academics, but it shouldn't take me long to catch up. And you still didn't tell me what is wrong with being aware of the value of exercise from a young age. I would lift weights as a teen instead of an old man who is probably just doing this because he's trying to feel young again

If quantum uncertainty is proved true via observable irregularities with memory, it would at least confirm alternate timeline theory. I wouldn't have to kill myself to undo existence.

I would avoid certain people and embrace others just to see what if

I would become the most bizarre eoffer ever just because of my uncanny observational skills.