I rarely drink coffee, but when I do I tend to make too much. I did a little research to find out what I could do with my left over coffee, and I learned you can turn brewed coffee into a fast-acting fertilizer by diluting the coffee with water.
The sites I visited suggested diluting the brewed coffee down to 1 part coffee to 4 parts water. I have been using a weaker solution (1:10 or 1:5 coffee-to-water ratio). I have about 1 to 2 cups of coffee left over so I just fill the coffee pot with water until it reaches the 10 cup mark, and use that to water my plants. Brewed coffee is acidic so I use this solution to water my acidic loving plants (e.g., blueberries, azaleas, hydrangea, roses, etc.).
I did not notice dramatic changes in my blueberries, but my 1 year old hydrangea noticeably perked up within a day after the first application.
Note: You should refrain from using this solution too often, especially if you use this on non-acidic loving plants.