Observing for ~100 hours is the “minimum” requirement set by most schools. I spoke to the Dean of Nova a few years back who first gave me that number. He also told me any more shadow experience than that is a “concern” to them. At that point they expect you to become actively involved with dentistry by working as a dental assistant, taking dental assisting classes, taking other dental related classes, volunteering at clinics, etc.
One thing we offer at UCI to our members are tooth-waxing classes and denture making classes. These classes teach you oral and tooth anatomy from a hands on approach. The course is taught by a UCLA dental school bench instructor. Dental engagement like this show dental schools that you have physically worked with something dental related and that you are adroit (dexterous/artistic/mechanical) nature. This is where dentistry mainly differs from medical school. They want to see that you know how to use your hands.