Put concrete blocks left over from constructing a wall to use in the garden. The blocks are somewhat heavy, which means they're stable. When building something that's more than three blocks high you will need a cement foundation and to mortar the blocks together. Paint the blocks to match the house or fence or choose a contrasting color.
Use the blocks as a mowing edge between the lawn and flower beds. Dig a trench that is as deep as the block, less 1/2 inch. For example if the block is 8 inches tall, the trench should be 7-1/2 inches deep. Bury the blocks so they are just slightly above the soil line. When mowing use the row of blocks as a guide for the lawn mower's wheel. You'll cut down on manually clipping the grass up against the blocks or using weed eater.
Dig a trench that's half as deep as the block. In the case of an 8-inch block that would be 4 inches. Bury the blocks with the opening in the tops up and visible. Fill the openings with soil. Plant flower seeds in the openings. Good choices are low growing flowers like lobelia, moss rose or sweet alyssum. Another option is to use two rows of blocks with a 6 inch gap between them. Fill the gap with soil, and plant flowers in it.
Raised beds solve the problem of bad drainage and poor soil. Put a layer of newspaper or shredded paper where the bed is to be located. The paper kills any grass or weedst and adds organic material to the soil as it decomposes. Stack the blocks so the seams are not directly above each other. Stagger the joint lines. Fill the bed with potting soil or a mixture of one-third potting soil, one-third garden soil and one-third compost. Hose down the bed when it's half full of soil to settle it and again when it's full.
Concrete blocks have an opening in the top that's just right for starting seedlings. The blocks are heavy and can't be moved easily, so locate them in a warm area of the garden. Fill with potting soil. Plant the seeds two to a section. The blocks retain heat; seeds germinate faster when the temperature of the soil is warm. When the seedlings are ready to transplant into the garden, just lift the block up. The seedling should easily slip out and remain on the ground.