Coffee grounds are an effective substrate for growing some mushrooms, and they're a lot easier for city dwellers to get than horse manure.
This book describes the procedure:
Organic mushroom farming and mycoremediation : simple to advanced and experimental techniques for indoor and outdoor cultivation by Tradd Cotter
[It's a book for the adventuresome - he also grows oyster mushrooms in used pizza boxes, and devotes one chapter to the very difficult growing of morels]