Use cement blocks to edge a garden, and fill the blocks' holes with soil and plant to finish -- low-growing plants like creeping thyme, moss roses or strawberries work well and will not grow high enough to shade or otherwise interfere with garden plants.
Get the kids to make some with me. Raised garden bed made out of mosaic cinder blocks. I think I may just do THIS for our garden this year! Turn them on their sides and plant the seedlings in the open holes...perfect containment, no room for weeds! ;)