One popular way to organize small groups is by topic. This allows people to choose a group based on a topic of study that they are interested in learning more about. These groups will attract people in different stages of life, which is helpful and bring in different perspectives and can generate good conversation. A few ideas for small group themes are:

Book of the Bible studies:  Every book of the Bible can be made into a study, the Gospels are especially interesting since it gives you a chance to dive into the life of Jesus, which is the best example we can follow.

Theme of the Bible studies:  There are a lot of great themes that people always want to learn more about.  You'll take a theme like forgiveness, and each week go through various Bible passages that cover the topic.