We used to critique our Montessori equipment with our directress who was a developmental child psychologist. She suggested once that the solid geometric shapes were wonderful for tactile experiences and vocabulary for geometry. Her improvement for the shapes would be that they were clear and you could measure the volume of each shape.
This is something she would have liked! The lids can be removed. Measure volume using millet in a measuring cup and grade the shapes from the largest volume to the smallest volume. It's a great Montessori lesson.