I wish I was fluent in another language: given the time I'd like to study German, French, Spanish, Hebrew, Chinese, Japanese, Hindi, and/or Arabic, in roughly that order.
I wish I had learned to play an instrument and/or read music.
There are huge gaps in my literature background: I've never read Joyce, Faulkner, Jane Austen, Dickens, any of the great Russian novels. I don't need or want a class for any of these, but it might be fun.
I took a lot of dance classes in college, but I'd like to keep up with it. I also dropped out of an 8am karate class my first semester freshman year after missing every class except the first one, but I'd like to study it again.
I took a 300-level class on Marx and Marxism, also as a first-semester freshman (everything was filled by the time I registered), and had no business whatsoever being in it. I'd like to do it over. Ditto for my one science requirement: it was Earth Science with this horrible old creep. I actually enjoy science, so I wish I had taken something with a better teacher.
I suppose I could do with some business classes, but these Harvard Business School Pocket Mentor books are reinforcing the idea that I didn't miss much.
Here's a tidbit: I somehow weaseled through four years of college without ever taking a freshman Expository Writing class but then ended up teaching that same subject at Rutgers for six years, which was infinitely worse. Karma's a bitch!