A language, of course, first of all because you have to speak another language to be well-rounded and secondly because being bilingual makes magic much, much easier. Jillian takes French, because there are bits in the Lord Peter books she can’t read, and because most magicians are either French, Russian or Indian. She doesn’t like Russian (too dour) and most Indian papers are still written in English, so, French.
And astronomy, and at least one science beyond the introductory course (she does physics), and basic sociology, though more is better. She crams in a literature course here and there, and plays third clarinet in the orchestra, too, to relax. She doesn’t get a lot of sleep, and tends to drink too much on the weekends and sleep with people maybe she wouldn’t normally, but that’s fine, that’s what college is for.
Oh, and math, at least up through the 200 level. She likes math, the cleverness of proofs, the way simple rules complicate themselves, but quickly realizes she isn’t a mathematician. It’s the difference between being a craftsman and being an artist: she gets the concepts well enough, but there’s no voice in her singing equations. She sits in the second row and envies mightily the front-row students their ardency and poetry.
It all comes together in the 300 level courses, though. She is of a thousand lives lived, and commands the tongue of the birds. She swims in Hali beside the shattered towers of Carcosa. Plants grow at her command, and water flows. She holds forces in her head in perfect balance; like Maxwell’s demon she works without working.