I'd like to also know what to use for retaining walls! I would just caution you about weep tubes.. if those are the holes you are talking about you might not want to block those! ;-)

Consider planting just behind the block wall, and filling the tops of the empty block with some soil so plants would root into it. The holes themselves are rather small for most plants.

Great "drippers" are phlox subulata, candytuft, setrectia if you like the look of "outer space" plants, dianthus, smaller ivies (Plant those in the holes!!!), like variegated needle point ivy (cut it before it roots below, although mine never gets that far if it's left without extra water), hardy iceplant, Wire Vine Muehlenbeckia axillaris(which can be sort of invasive so I'd also plant it in the hole). It will root where it touches the ground, but it would serve your purpose well!, creeping fig... just a few ideas!

GGG

The holes are at the top of the wall. They are the tops of the concrete blocks. I figured out what I am going to do from the advice of someone on the accroutements forum. I have a nursery ordering me some creeping fig. It will affix itself to the block and cover the wall in greenery. I might still plant something in the holes at the top for color until the fig vines grow. I might try the iceplant b/c I love the look of it and it will hang over the sides. I don't have much luck with Dianthus (go figure). Or maybe I could plant some Purple Wave petunias in there for the season. Thanks for the suggestions.

Creeping fig will completely cover your wall.

How fast does creeping fig grow?