Use some of the concrete pieces to make a raised flower bed around a tree. To start this, you'll need to remove the layer of sod so that the first layer of concrete pieces are on the dirt surface. As you remove the layer of sod with the shovel, be sure and carefully make a circle around the tree.
Start the first level by laying a piece of the concrete into place. Make sure the concrete piece is upside down, meaning that the rough surface faces up. This makes for a much more interesting display when it's all done.
Fit the broken pieces together like a puzzle as you work your way around the circle. Stomping on the broken piece of concrete helps to level it into place. You may also have to dig a little dirt out from under it. Remember, it doesn't have to be perfectly level to make a nice flower bed wall.
Stagger the next row of broken concrete pieces to lay across two of the bottom row pieces. Build the second row just as you did the first row. You may have to break the larger pieces, or sift through the pile to find ones that fit. To break a piece, just carefully drop it on another larger piece and it will break. This will be a trial and error process to get the right size.
Depending on your taste, you can leave the retaining wall at two rows are do a third row, but don't go higher than that or the wall will be unstable. Fill the circle with as much additional dirt as you'd like and start planting flowers.
Create a path through the garden or enhance one that's already there. Use the broken concrete blocks for stepping stones, and fill in between them with river rock, sand or mulch, whichever you prefer.
To enhance a rock bed that you already have, dig the river rock, sand or mulch out in places and replace it with a piece of the broken concrete. Put the broken concrete in upside down so that the rocks in it show and blend with the rock bed. You can build a small patio using these same techniques.
Hose the concrete sections off when you are done. Your new stepping stones are easy to mow around and require minimal upkeep. Just wash them down with a hose occasionally.