Depends on your goals. You can just get a fun geometry you want to work with, then try to mesh it, setup and run the case. It's already a good practical exercise which will push you to think about what you want to do and how to achieve it (meshing the geometry, setting up boundary conditions, etc) Or you can try to do some validation: on a topic you like find a simple case with reference data / paper. Reproduce the case from scratch on OpenFOAM and see if you can match your results with the reference.