One of the most frustrating things as a band director, is when a student doesn't have their instrument. This is a tool for students to reflect on why they don't have their instrument, what they will do next time and how they will use their class time wisely while they don't have their instrument.
I have a binder set up and divided by class. Each student has their own page. When they come in without their instrument, they get the binder and make an entry on their page.
This is also a great tool to have during Parent/Teacher conferences to share how often they forget their instrument, and their excuses as to why.