Hi Christoph,
I encountered a lot of these "limits" in the document generator. Going deeper than 2 levels in the project hirarchy is not that easy

Since you are using the new version 10, you may use "template fragments". Using two template fragments, one for the use cases and one for the activities, you can set separate filters ("exclude details for...") for the two types of elements. To define a template fragments, you have to tick the box "Template Fragment" in the template options under the tab "Document options".
Follow these steps:1. Define a template for the use cases with the following sections (TestUseCaseTmpl):
package >
element >
Use Case Name: {Element.Name}
--> and then all your needed data for the use case details
< element
< package2. Define a template for the activity with the following sections (TestActTmpl):
package >
element >
diagram >
{Diagram.DiagramImg}
Figure {Diagram.Figure}: {Diagram.Name}
< diagram
< element
< package3. Define a "real" template, where the two template fragments are referenced:
package >
element >
{Template - TestUseCaseTmpl}
{Template - TestActTmpl}
child elements >
< child elements
< element< package
One thing you will definitely stumple upon: the line breaks between the two template fragments; you will have to experiment with them to get the best result from the templates

You can insert the references by right-clicking within the "element"-section and there selecting "Insert Template". More information on the fragments you can find in the EA help.
I hope that helps.
If you still have questions, just ask!
Jonathan