I miss long summers with nothing I *had* to do. Being able to just hang out with friends, read a book, play a game, or do whatever, whenever. I miss the way I liked school when I was younger, and even though I hated high school, I miss the casual confidence I had when it came to "success" in school. It was, relatively, a low-stress time.
Mostly, I miss dreams that were uncontaminated with reality and practicality. That and really being able to believe that I'd live to see (and participate in) some sort of colonial-style settlement (moon, Mars, orbital space station, etc.). Leave behind the cruft of societal expectations and start from scratch.