Here's a fun idea I actually used with my church choir a couple of years ago, but it would also work great at a group piano lesson. On a table, place a small paper cup for each person in attendance. Randomly place 1-8 M&Ms or Skittles in each cup. Everything should be done ahead of time. Some will have 1, some will have 5, etc. No patterns here! Then tell each student to take one cup and tell them NOT to eat the candy. I used this game one time to help everyone to get to know each other a bit. Depending upon the number of candies in their cup, they had to tell us that many things about themselves.
Another way I used this was to correlate the numbers to a question. For example, if they picked a cup with 2 M&Ms, they had to answer question #2. Using this with piano students, you could have them respond to things like, 1 - Name the Note, 2 - Tap out a rhythm, 3 - Guess the Composer, 4 - Name the term, 5 - Play a scale, 6 - A listening exercise...You get the idea! The key is not to show them the questions/activities until after everyone has a cup of candy. In fact, I didn't even tell them what we were going to do at first. It made it all the more mysterious! This is a super fun activity! I'm sure you can think of lots of other ideas that will work also!
What was funny with the adults was identifying who took the cups with the most candy! Kids will probably go for more, so consider making the higher numbers more challenging. Of course when you're all done with the activity, everyone gets to eat what's in their cup! Super Fun!