I would major in Museum Studies/Historic Preservation NOT International Studies: Soviet Union and Eastern Europe.
I might have been better off majorin in illustration instead of Visual and Environmental Studies, but mostly I’ve enjoyed being an architect.
My younger son thinks he should have majored in History instead of IR. He’d have ended up in the same place, but probably with better grades.
I’ll play.
I would have majored in accounting (which I ended up doing professionally) or marketing instead of biology.
I would have majored in biology, in spite of its difficulties (for me, anyway), instead of comparative literature.
I would major in the same but my dentist wishes he majored in something humanity.
Actually, I’m happy with the choices I’ve made.
I’m a high school math teacher. I love what I do (well, most periods anyway 
) I’m good at it; I think kids are glad to see my name on their schedules in the fall, and glad that they’ve had me in June.
Of course, if I could go back in time, I would have invested heavily in Apple or Microsoft, but that’s a different thread.
It has been fun looking at all the different majors available while D’16 has been sending in her applications. I’ve run across many that make me say “OHH, that sound fun./interesting”. Several that make me say “that’s a major?” , but then again I am sure that is exactly what people said about my major back in the day!
Accounting, which is makes up a good portion of what I spend my days doing, and then go into forensic accounting. My Econ degree is fine for financial analysis, but after all these years it would be nice to have something that gives real options. I love the hunt.
I’ve always said that if I won the lotto and didn’t have to work I’d go back and study history.
Fashion Merchandising instead of accounting.
Accounting instead of engineering! Our accountant now charges over 100% more an hour than he did in 1999, while we have gone up only 38%. And what we do is a lot more complicated and risky from a liability standpoint.
My engineering grad regrets not doing accounting, perhaps with a side of math or CS, for the same reasons that @MaineLonghorn cites.
My change is what I would have done with my career. The happiest period of my life was when I was an undergrad in EE and I wish I had gone in a career that used those skills at least to some extent. Turns out the only place I did any EE was with my kids when they were young. DS was presumably influenced enough that he did pick up a double major including EE, and even interned in Intel for a summer, but he abandoned the field too in his career.
Polisci (IR concentration) and journalism major, but I really loved history. Would have considered had I any idea back then that one could be a lawyer without appearing in court.
I love the big picture aspect of history/polisci/anthropology, but where I have done well in my career is with highly detailed work (editing, project management, pension accounting, knowing regs and procedures). My fabric art is really detailed, too.
I enjoyed the Computer Science class that I took as an upperclassman. I found the assignments fun and didn’t mind spending hours in the computer lab. Throughout the course, I was telling myself, “This is how I think.” The final was a programming project. I chose that hardest one and aced it. Would have like to have taken more CS courses, but I had to finish up my major.
Biology/botany, psychology, or math education instead of English lit./ESL.
I like what I do, and have loved being an art teacher (and am very good at it), but I often think that I have an engineering type of mind - I loved science and art. I used to put together models, and my favorite presents were “build this!” types of items, such as “build your own transistor radio!” Math was always a challenge for me, but I loved “figuring things out” - I wish my parents had given me a math tutor, and exposed me to engineering - as a girl, it must have been not on their radar in the late 70’s. I’ve been recently trying to teach myself how to code, because I think that coding coupled with a visual mind and sound logic might be a great second income stream.
Instead of pondering about it, I just did it and switched from EE to CS. As a reward - a fun job that I love. Just do it! We live only once, you should enjoy what you are doing.
Occupational therapy instead of sociology. Better fallback or moonlighting job than I have now. And maybe even instead of my M.Divinity.
Career wise, OT or PT rather than nursing. I was so clueless. In my heart though, geology or botany fit my interests the best, and I would have reveled in the details and lifestyle that goes with an nature focused career.
Many say history. Did you love it as an 18 year old, or did your interest grow over time? I was bored by history when young, now I find it fascinating.
landscape architecture, or I would have gone to culinary school.