First consider how to deal with the falling water as it can beat plants down to the ground. You might try putting a river rock or other stone strip where the water lands after coming off of the roof to take the brunt of the falling water. the use the rest of the soil surface for garden plants. This area could even be a walkway so no space is lost.
Mix in lots of compost to improve the water and nutrient holding capacity of your sandy soil. If the area is sunny you can grow lots of blooming plants. If it is shady most of the color will need to come from colorful foliage. I you want to submit another question to the Home Depot website to clarify what type of garden (vegetable or flowers) I can provide more specific plant suggestions.
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