Definitely shadow a dentist, volunteer your services to him/her if you can. The dentist may allow you to assist him/her with the procedures, which is pretty cool. A letter of rec from the dentist would look good as well.
But in addition to this, I think any volunteering activity where you are serving your community in some way would be excellent, make sure its something that you will enjoy and are curious about yourself, something that you can gain growing experiences from. It doesn't have to be dental related, just an activity that illustrates that you have the heart to give back to others. Adcoms like to know that the students they're taking in, have the dedication for a patient based/public health service such as dentistry. In addition, it adds a lot of interview material you can discuss, that further aids the adcoms in getting to know you. I did high school mentoring and hypertension screenings in underprivileged areas. I also was a counselor for a summercamp that brings underprivileged children free of charge, up to the mountains to camp. I very much enjoyed my experiences and grew as a person because of them. I enjoyed meeting people from different backgrounds, and improved upon my public speaking and leadership skills. -good luck