Volunteering anywhere is fine. There is no minimal requirement that you volunteer at all, however, if you feel you must volunteer in order to improve your application (rather than out of the kindness of your heart) you may as well volunteer doing something you want to do. Consider homeless shelters, living facilities for the elderly or mentally or physically handicapped, preschools, high schools, middle schools, churches, etc. It doesn't have to be dentistry or medically related.
Make sure to simply do at least a minimal of 30 hours of shadowing or observation with a dentist as that is the highest minimal # I've seen for any school.