DatePicker with the ability to choose a span.

Answer 1

The WPF Toolkit has a Calendar/DatePicker control that could be a suitable base for you to modify into an elaborate date range picker.

I would start with the DatePicker Generic.xaml file from the WPF Toolkit distribution, and examine what they have done with the DatePicker control template. The interesting section for you will be around the area that PART_TextBox and PART_Button is used. They indeed use a combination of grids, textbox, button, and popup controls.

Hope that helps!