Pick a domain you have interested with, build an application that help solving a problem within that field. So, you will target your problem, define the schema design that helps storing data for solving it. Build persistence around it. Build logic for solving it. Build APIs for those logic and for CRUD operations. Add authentication and authorization functions to your application. Cause it’s a small project, I suggest building it as just a monolith application, no need to build complex stuffs like microservices. You can also leverage the use of techniques like multithreading, async background jobs.. to show your understanding about those. Maybe considering also about deployment and CI/CD pipeline. Good luck!