Most dental schools will stipulate how much work experience you have to do on their website. What I thought was really key in getting accepted was the fact that I had done a range of different areas of work experience. I had been in general practice, with a dental technician, and also in maxfax surgery. This not only meant that my personal statement was more interesting as a result, but it also gave me an advantage when it came to the interview stage.
There is no set list of activities that you should do, but my advice would be to make your existing activities bend to your advantage. For example, I used to be involved in a drama club at school, which sounds like it isn't at all related to dentistry, however in my personal statement, I showed that being involved in a school production meant I could work to a deadline, and also perform in a group and under pressure. Play to your strengths, and good luck!