Interesting things to do during a gap year after getting undergraduate degree

Hello, all. My sibling asked my for advice recently, and I’m making this post on his behalf.

Said sibling is currently currently has one more year to go at MIT before he gets his bachelors degree in Mathematics/​CS. He is also enrolled in a 5-year masters program, so he will need one more year after that to finish a Masters, after which he anticipates getting a job somewhere the CS Industry /​ Finance /​ Academia. Anyway, he is interested in taking a gap year after finishing his Bachelors to pick up some novel experiences, and trying something different from what he has been doing already and plans to do after graduation.

Right now, he is in the brainstorming stage, and is looking for ideas. Note that he is not opposed to getting a job or something of the like—as long as its a different experience that what he would get working for a large software company, or a hedge fund, or something of the like. Financially, he does need to earn enough to live on (this isn’t quite a vacation), but he isn’t worried about money aside from that (so the “money” constraint only needs to be satisficed, not optimized.) With that said, what are some things that he might consider doing?