Chopin, Mazurka in A Minor, Op. 67 No. 4 (Posthumous)/User
It's a longish piece, and there are several challenges. Student must grasp the Waltz bass, and carefully control the Oom and the Pah Pah, while keeping a clear cantabile above. Through in a few upper neighbor ornaments, and you've got yourself a solid, late-intermediate, early advanced piece.