I find this question so tough. Just to be studying it, probably art history. (But lots of others that intrigue me -- philosophy, history, semantics, computer science.) If I were thinking of a different profession, probably veterinary or medical school.

This is actually a question that we frequently talk about around here, living as we do book-ended between two top notch universities. We take extension courses through one university, and my husband has taken more serious, degree-oriented classes at the other with the goal of one day maybe pursuing a long-held dream of his.
While he is clear on his future studies, I am not nearly so. If I were to pursue a second career, I would finish a master's in public health. If I just wanted to take classes for the rest of my life with no clear goal other than a purely intellectual one - I find Chinese history fascinating.

I actually did go back to university to study. Took me 1.5 years and 20 hrs a week. On the intersection of IT and business administration (I think that's what it's called in English).
Nowadays there are also options in online courses. Some of them are quite challenging. I followed a few and learned a lot. Following some right now actually. On artificial intelligence.
All of this is work (IT) related.

If not IT/work related I'm not sure what I'd study. Maybe music theory/composition while studying a polyphonic instrument like the guitar/piano at the same time.
Another option would be to choose some mathematical subject and get proficient in it.

If we're talking about a different career maybe I'd study some different engineering area. Like fuel related. Or biochemistry.

Don't mention the Harvard extension programme.

Languages and translation.

That's too vague! What language?

As many as I could. Probably mostly European. Since I'm far too old to go back to school I don't really need to be precise. :P

Medicine, I think. I'm thinking of a career change away from translation - all that time alone isn't good for me.

If we're talking just-for-the-hell-of-it study, I'd LOVE to learn Hungarian and study the literature and culture.

Carpentry, cabinetry, luthiery. French cuisine, butchery & charcuterie. Tailoring, dress & costume design. Piano, tabla, sanshin, oud. Acting, singing, public speaking. Botany, landscaping, formal & informal gardening. Judo, aikido, tai chi. Painting, drawing, sculpture, pottery. Rifle and bow marksmanship.

Anything that makes way more money than I'm making now, like accounting or nursing.

Geology.

I would never return to school. I never learned anything in school that stuck with me, and I hated the formality & conventionality of it. If I had that same $ and time, I'd

- go to Burning Man,
- go work with float makers for the Rose parade
- apprentice with a skilled shoe cobbler
- " with a solar power design & installation company
- " with a classic care restoration shop

And probably a bunch of other things.

care = car

Gemology and jewelry design

Architectural history probably.

If it was for a new career, then medicine to ultimately become an OB-GYN.

If it was just for fun, then any sort of world history.

classics, Greek and Latin.

philosophy

littrachaw

Genetics, or astronomy. Research of some kind. I miss studying science. It would mean starting from scratch, though. Alas, I think I'm too old for such a sharp turn now.

On a more practical note, maybe a PhD in clinical psychology, work with adolescents. (I have a B.A. in psychology from Michigan.)

I could also do a PhD in creative writing (I have an MFA). Maybe if my crazy principal ever finds a way to fire me (he's been somewhat human lately, but I don't trust it.) Pick up a scholarship and teaching stipend somewhere. Vegas, Denver, Houston...

Jazz clarinet would be cool, too (my other major at Michigan was music). I play classical and would love to learn jazz. Of course, there are some great music schools around here, but the cost is prohibitive. And again, I'm so old.

What I need is to live to 300.

Either your mom, or mathematics.

Either your mom, or mathematics.

When I'm done with the current one, I think about maybe a nonprofit MBA, or a history MA.

Absolutely IT, probably DBA. A full on study of yoga, with a goal of daily independent practice to take me through the rest of my life would be awesome, too.

Just because? History for sure. For career purposes? I don't really think it matters - as long as you have a degree, you have an advantage over everyone that doesn't in any job. Probably CS of some sort - I'd love to be able to make that magic happen when lines of code turn into something that actually does something.

I'd love to actually finish my CS degree since that's the field in which I've been working since the late 1990s, and I pick away at it as I can. I started out as a CS major and then went for the clearly more useful English degree.

I get frustrated because I'm constantly told it's not worth it as I should be looking at management, which isn't really what I want to do. I haven't figured out entirely if it's because of my age, or that I am am a woman. I used to work for a company in which all the men became System Analysts and all the women became Project Managers whether they were technical or business oriented.

In any event regardless of the jobs available to me, it would be good for *me* to have that degree and to have that basis.

Outside of work stuff, I'd love to go back and study languages. My French speaking ability has degraded quite a bit, and I'd love to become more fluent in Spanish. Then, Japanese. I've been studying French via duolingo, some books, some music CDs and reading Le Monde each day. My coworker who used to live in the Netherlands is teaching me some Dutch so I can say at least a few polite things when I'm there in the Fall. I definitely love being able to think and talk in other languages. But it's a little harder when you don't have an actual person to chat with so classes would be good (and all seem to be scheduled during the work day, bah. And none on Coursera whenever I look).

And that said, really, I would be completely content to just be taking classes in nearly any subject.

Ha Sil. On CS: I've been reading on the connection between types (i.e. classes) and logic today. Hard core CS!

It's easy to please the Dutch with some phrases. Since nobody speaks the language. It's a bit harder to get practice though. Since everybody switches to English automatically as soon as they hear your accent.
What brings you to NL?

I've gone back to school a couple of times and don't want to do it again, but I'm with chewatadistance on the apprenticeships. The things I want to learn more about now are more hands on, bookbinding, furniture making.

I'd like to study photography. I'd also like to (re)learn French (I studied it at degree level many years ago), but I'd like to do that by living in France rather than by sitting in a classroom.

Theory.

European history

Japanese

jouke - it's my sister's birthday so we are meeting her in Amsterdam for a good wander. We're there for a few days (likely just enough to find out how much we are missing so we plan a longer visit back). I'm really loking forward to it, but I need to remember to post a question to AskMeFi because all the restaurants my research has turned up are closed on Sundays, which is her birthday. This will be my first trip to the Netherlands.

I'm glad that all my sister wanted to do for her "round number" birthday is to travel with me. I'm super excited! We're headed to Nice after that and then northern Spain.

Amsterdam is very walkable. So that should be fun.
Here's a list of highly rated restaurants in A'dam. They don't have a filter for opening days unfortunately.

Language and translation for me too.

Also auto repair/restoration.

Linguistics. Had I taken the intro class freshman or sophomore year, I might have minored in it instead of math.

I'd be interested in learning electrician type skills since all I can do is install outlets, switches and wire lamps and other basic homeowner stuff.

I already do all of the plumbing stuff and can get into enough trouble in the carpentry realm.

If you're talking about impractical, I'd like to apprentice with a good horse trainer.