There’s no magic formula. Build something relevant to what you want to learn about, or relevant to the job you’re seeking.
To keep with the analogy, if a chef wants to work at a Mexican restaurant, burritos and tacos are a good thing to practice.
If a developer wants to work as a frontend developer, building an interesting and interactive interface is likely a good option. If you’re targeting a specific company, try re-building part of their public website. Bonus if you add a feature they don’t yet have that you believe would be interesting for their customers. That can be your “something”.
That would definitely stand out in an interview.
As far as “just enough”… again, it’s not a secret code word. If you can make a half decent frontend (for a frontend position), just start applying to jobs.
Do you have any suggestions as to what kinds of apps and knowledge would be sufficient?
For Frontend: Build a forum (like FCC’s!)
For backend: Build an API that you think would power a website you use commonly (FCC again?)
Infrastructure engineer: Try building a simple API that runs across two servers, then automate the scaling of it to work on 10 servers, then back to 2, and see how it works.
These are just ideas off the top of my head actually. There really isn’t a “correct” project. Build something interesting to you that lines up with a job you’re after.
If you have a specific discipline or industry in mind let me know and I can try to give info on what a developer in that field might do day to day.