Some suggestions:
-Volunteer fire department (probably wouldn't work with your tight schedule)
-Local health clinics for homeless or poor families (may not be dental, but it shows effort)
-Habitat for Humanity (once again, not dental, but it shows that you are trying) and you can do administrative/fund raising work for them which allows for a more flexible time schedule and much more consistent work than if you do only site work.
-Local child vaccination program ("Well child" clinics)
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The three dental schools I talked to said that the preferred dental or medical related experience, but that they also value any experience that shows that you genuinely care about people and are willing to give of your time. The exact way the first school's representative put it was "We want to see effort, consistency, dedication and honest concern for those less fortunate."
They seemed (from what I was told and could assess) to put more emphasis on longevity in a given volunteer slot than on the nature of the slot- one of the admissions officers told me that my background as a volunteer firefighter over the past 8 years would be far more highly regarded than someone who has spent a couple of days working in a free dental clinic, because it shows that I simply was not there trying to get a recommendation letter for school or to pad my application.
Pick something you think you would enjoy doing and do it with great zeal and I think it will show and that the admissions committee will see that. I hope this helps.....