Sciencewise: extra physics, math, or biology courses are always helpful. Linear algebra or diff-eq are probably both good options, but if you're more interested in biochem or organic chem, you probably won't need that stuff. Computer science (programming) is something I definitely wish I had taken.
Non-science: as a career scientist, you will do a lot of writing, so any elective that helps you out with this skill will definitely be useful, particularly if your writing skills aren't so hot.
Other than that, I'd say just take something that interests you. You're going to be focusing on chemistry for the rest of your life. Take the opportunity to learn something new or different, even if it won't help you directly. In undergrad I "minored" in both philosophy and history. Did these courses help me in my chemistry career? Hard to say, but they definitely gave me a more rounded education and helped develop my critical thinking and writing skills. Beyond that, you'll probably never have the opportunity again to explore such formal coursework, so you should take advantage if you can. My two cents.
(I put "minored" in quotes because my undergrad didn't have official minors until about the time I graduated. But I took four to six semesters of coursework in both of these departments by the time I graduated.)