In the book the sisters suggest using a set of chimes to signal your students back to the meeting area for check-in. They say the balanced melody of chimes is different enough to grab the attention of your kinesthetic children while unobtrusive enough to not upset your auditory learners.
I purchased a chime wand from Music Wands to use when signaling my students back for check-in. You lightly tap it against something and it chimes. It is perfect! The wands come in a wide variety of shapes and you can't beat the price.
Don't forget to introduce your signal to your students before using it. Make an anchor chart for what it will look and sound like when you use the signal.