Ask HN: What can an undergraduate work on over summer without an internship?

Hi HN. As the title suggests, a typical CS undergraduate (freshman) is hoping to looking for some ways to boost my technical/non-technical skills over the summer. I know that many may suggest looking for an internship, but due to personal reasons I am not looking to pursue one for this summer alone. As such, I was hoping anyone could possibly suggest interesting projects I could work on which most importantly balances practicality with interest (i.e. should ideally help to gain some relevant 'workforce' skills, while being something which I may not have the chance to in an actual job).

Suggestions such as diving into operating systems, learning about distributed systems are appreciated, but would hopefully take a backseat as I believe I will have time to dive deep into them when they come around in my curriculum over the next few years. A detailed suggestion in a past thread [1] suggested learning/implementing Git from scratch. I like this idea and am planning on at least gaining a deeper than average understanding of Git, but was looking for more variety across different domains other than version control systems.

Any advice would be appreciated. Thanks!

[1] https://news.ycombinator.com/item?id=19642190