Bach's Prelude in C minor is a wonderful piece for late-beginner/early-intermediate pianists, full of energy and character. Although originally written for the lute, this piece fits the piano very well exploring both percussive and melodic qualities of this amazing instrument. Both hands never play together, but the coordination to keep the notes moving evenly might be one of the biggest challenges here. Memorization is also tricky, because the harmony is very chromatic. All in all, a great challenge if you're ready for it!