creeping jenny will fill in and hang down over the edge. might even overwinter to grow slowly again next year

A few options:

Huchera- it comes in a number of different colors from deep marroon, to peach, to chartruse green.

Euphorbia- there are some smaller varieties available. They will stay green through the winter and fall.

Lettuces or cabbages would look nice, though they may need more sun.

Pansies, snapdragons and pinks are all nice.

Ivy, creeping jenny or other trailing plants would make a nice addition to the mix.

One idea is to put in a perennial vine that will transition spring,summer and winter which will trail over the sides of the containers. Creeping Jenny is my favorite but this only really looks great summers. example:http://www.aboutvines.com/vinca_vine.shtml
The Kales would make a colorful addition for winter and Mustard Greens.

I like Vinca minor 'Illumination' as a cascader - it's evergreen, grows best without full blazing sun, and it takes a few years before it decides to take over the world at which time pruning will keep it under control.

As a filler I use Wire Vine, not really a vine but a tiny leaved ground cover Muehlenbeckia axillaris. If you can find the even tinier form Muehlenbeckia axillaris 'nana' it should work well. After a few seasons you have to cut these guys back, they can choke out just about anybody.

In the winter I use violas or pansies. I like to pair them with ornamental cabbage or Dusty Miller (the half tone pansies, with white edge and white top petals and wine red on the bottom half look great with the white leaves of Dusty Miller).

In the summer I would use Caladiums or begonias as long as your few hours of sun are not too harsh.

I second the wire plant. Great filler and tough. Stays evergreen in a protected location. I have some to trade at the fall swap if you want some.