Good C++ side projects to put on gitHub for a beginner?

So I lied on my resume that I knew C++ and now have an interview in 10 days with a big company for a WiFi testing position. I have now learned the C++ syntax I already know data structures etc. in Java, R, Python and HTML,CSS and JS. Have made games and stuff before that I have in my gitHub. But what's a good app I can make with C++ ? I've heard GUI apps aren't really big in C++ ?

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edit: it's only a student level job(I'm still in school and am required to complete like 4 of these 5 month long placements to graduate). I've done a few of these before the questions are usually non-technical and as long as they have evidence of you having coded on your gitHub they're good with you.