Well, that's certainly quite a question, and not one with an easy answer.
I think the most common activity to all planners is land use planning and regulation. This includes zoning land for different uses and different types of development, reviewing the subdivision of land for new development, setting land use and built environment policies, and assisting in developing land efficiently and effectively. Most planners (urban or not) probably do something in this area.
Beyond this, planners can get involved in a multitude of different issues, such as transportation, housing, economic development, mediation and facilitiation, public administration, historic preservation, social policy, natural resources, and on and on.
Planners are a very diverse bunch of professionals, but dealing with issues of land use and development is something the vast majority of them all do.
What else can people think of that planners have in common?