When I was 18, I wish....
My friend Ben Casnocha just turned 18 (thank God I'm not yet old enough to be his dad...when I start having friends that are young enough to be my kids, then I'll really freak out), and as a birthday present, asked his geriatric friends to tell him what they most regretted either doing or not doing as an 18-year-old.
My answer was that I regretted not doing a quarter overseas, probably in Oxford or Salamanca. At the time, I was busy with school, and didn't think that I could afford a term away, given the heavy requirements of double-majoring.
In retrospect, I truly regret it. As it turns out, I recently found out that my friend Alysia Andrikopoulous was in my same position, and got a dispensation from the Stanford Product Design department to skip one of the core classes and still graduate.
Plus, back in 1993, the world was an incredible place. Communism and fallen, and we hadn't yet experienced the full horrors of ethnic cleansing, genocide, and terrorism. For a brief time, anything seemed possible.
Alas, that world is gone forever, and I shall always wonder what it would have been like to experience it.