Two major issues in the public school system is cleanliness and learning math. How about we solve both at one time?
First you need two hand sanitizer bottles of different 3D forms like so,
The goal is to estimate the volume by number of hand washes. First measure the amount used per single hand wash (you wash your hands while the class watches) then take turns. Next, have tally sheet posted on the wall near the hand sanitizer. Once the final squirt is complete, have the students tally up its volume. Continue this throughout the year with various shapes and sizes of hand wash or hand sanitizer.