That is very cool! I think moss painting on like hang up canvases might add a little life to an apartment; if one feels like doing graffiti, the sidewalk might be acceptable and legal. Also, a living wall or edible walls may also bring life into an urban apartment/home. Something like the Garden Tower would provide mobile vertical growing space, and Pintrest has a great display of many possible DIY Aquaponic systems that could be built.
Here is a picture of moss graffitti: