I think graph theory is undoubtedly one of the most important subjects for anyone working in the field of computer software. Speaking of which, back in university I made a website [1] that allows you to interactively draw graphs and animate a few algorithms on them, as my design project. Doesn't have a lot going for it feature-wise (no directed edges, no weights, just a few algorithms) but it was a fun project to work on. [2]
[1] https://ahmetsait.com/interactive-graph-theory/
[2] https://github.com/ahmetsait/interactive-graph-theory