* studied more
* explored tradeschool
* saved more
* told my parents i loved them more often
* sped less (i had a heavy foot as a young adult)
* drank less if at all
* took a few more chances, especially in the dating realm.
I could co-sign all of these, especially the last one. There's a line from one of Tom Selleck's Jesse Stone movies that I wish someone had told me at 18: "I'd rather regret the things I've done than the things I haven't done." The "what if I'd taken the chance" regrets are the worst by far.