Not applying myself to academics the first two years of college. I almost flunked out. I made the Dean's list my last 2 years of school (after I learned that I actually had to study), but I couldn't catch up my overall GPA. It's like 2.95 or something like that. Embarrassing, really. I wish I would have hit the books at 18 & 19.
Not marrying my wife earlier. We should have gotten married a year and a half earlier, but we bought into the social crap of "you should be engaged for xx amount of time". Life would have been soooo much less stressful if we had just gotten married.
Not taking enough pictures of life events through college. Of course, digital photography didn't exist, buts still, I wish I had taken more pictures. I don't have enough of my wife from back then. Sigh...
Not traveling more before I had kids. I am just now to the point where I can travel with my kids (and keep my sanity) now that they are tweeners. I wish I were seeing some of these places a second time rather than the first.