As I have written previously, the activities manager service now supports creating private activities.
But it is not exposed in the UI at the moment.
Now, that is not the only problem that the current settings module for Activities has. A few things have improved in the previous few versions, but there is always more room for improvement.
One of the common complaints was that the activities settings module does not actually allow you to create, modify or delete activities.
Both issues will be fixed in the next release of KActivities framework.