Activities include tasks and events.  Unfortuately Salesforce will not let us query the OpenActivities and ActivityHistory sObjects.  So you are forced to use two tables.  One will be tasks and the other will be events. 

Its all we can do. Sorry. 

Thank you Rob, Even when I use two objects Task and Event not all activities are listed. Do you know why this is?

We also have an issue with task list. It will not show tasks past a deactivated user. So if a deactivated completed a task on April 1, 2013, the task list will stop at that date. Happens on multiple records and object types.

The tasks may have been marked as “Archived” by salesforce.  Have you tried checking the “Query Deleted / Archived records” checkbox in the “Advanced” section of the model properties?

that worked!! thanks Ben