You asked what is Hens and Chicks. Look for Sempervivum. I buy mine at Squaw Mountain Gardens. This is what they look like. They do not die down in the winter you get different color at different seasons.
This ia a pot of them made into a topiary.
They would look good in a window box with some sedum.
Another idea....You could also try daylily Stella D'oro yellow bloom, there is also a Purple D'oro that blooms all summer these are two hardy plants that would survive a window box and for groundcover how about a variegated ivy.
Purple D'oro.
If you do H&C you will need to find other dry condition plants as suggested
If you do daylilies and ivy then you can add some annuals they will need more moisture.
You have two choice Sedum and Hens and Chicks or Dayliles and ivy and annual window box display.
That is my 2 cents LOL