100 Level history classes are interesting and super easy
As an engineer, I really liked the probability class I had to take. That was in part because the professor was awesome though, so your milage may vary.
How's your computer programming? I know a lot of MechE's try to get by without really learning it, but it's only an advantage. Also, once you get into industry, you might pivot into a role that needs programming, and having some further background will be beneficial. I have a MechE friend that happened to.
I also took a Science Fiction Literature class I enjoyed a lot. A lot of it turned out to be Western History as framed by the science fiction authors of the era, which was very interesting.