Something that might seem obvious but hasn't been pointed out yet... if you have cats, do they use litter boxes or go outside? I don't know about in cities, but people in my area aren't pleased when a 'present' is left somewhere in the garden. Much more acceptable if it's buried though, but British cats seem to be taken away from their mothers too early so that they haven't been taught to bury their excrements...
Lovely topic, I know, but a small grassed area could be useful for that...