If you want to schedule an activity automatically, you can use the activity_schedule() method of the mail.activity.mixin model. This will create the activity on a given due date. You can schedule the activity manually with the activity_schedule() method, as follows:

@api.model
def create(self, vals):
    res = super(LibraryBookRent, self).create(vals)
    if res.return_date:
        res.activity_schedule('mail.mail_activity_data_call', 
                               date_deadline=res.return_date)
    return res

This example will schedule call activity for the librarian whenever someone borrows a book. The deadline of the activity will be set as the return date of the book, so the librarian can make a call to the borrower on that date.