One thing I like to do is try to start conversations with people who believe very different things than I do. For a while I was doing weekly bible readings with a very interesting group of Christian people (I'm not Christian and have never been religious). It got heated. I was very challenged and forced to analyze my own beliefs and the presuppositions I brought to the table. I'm sure they felt that they had to do the same as well and I think we all became better people for it.