One of the things I try to do periodically with my students is rhythmic dictation. Here is a really simple worksheet that I designed for this purpose. I just have the student sit in my chair with a clipboard and pencil. Then I play rhythm patterns and have them notate what they hear. You can adapt the difficulty of the rhythm pattern according to the level of each student. This is also a great activity for weeks when students don’t get in a lot of practice, or accidentally show up without their piano books.