Sorry, that was not clear at all.
This is what I was looking for:
To fill in the Agenda with events in the future which are as well activities.
So every event is an activity.
And every activity will be filled in with the info from the event and only with the start date and not with the required stop date when you save the activity. (maybe not possible because of the calculation function).
I"m also looking for a possibility to send the customer automatically an mail or (even SMS) when the activity starts.
Maybe some existing plugins can be integrated.
The Stop date of an activity will be filled in on mobile devices, so it is necessary to have the activity list in Mobile format.
I hope this makes it clear.