These are the first two exercises of Giovanni Nakpil's mastering organic modeling course, that I did a while ago. It was highly recommended. The first task is to use only move, trim dynamic, and smooth to create a sphere from a cube, a cube from a sphere, then pull a cube out of a sphere. Its designed to be practice for getting you to move all around your model, a crucial habit in sculpting, as well as tuning the fidelity of how to use brushes.
Second is "the tentacle" - which was a continuation of the same practice. Start with only a sphere, and pull out a tentacle. It was just a free form design as you go organic skills practice. Its an excellent exercise for sharpening fundamentals and reminding you to constantly be moving.