Although you may think that almost every available space has already been colonised by advertising, for example the sky - skywriting, the walls above urinals, the sides of vehicles and the cityscape in general, the internet, media of every sort, films and tv shows (product placement), aid (for eg Anglicare and Worldvision and Caritas, who use philanthropy to advertise for their idea of "G-d") just to name a few, here are suggestions to better exploit currently wasted space.
a) Game reserves - each animal could be branded (literally and metaphorically) with the name of a corporate sponsor...eg this Zebra sponsoted by Investec, this rhino protectd by Falcoln Security, this tree proudly adopted by Anglo-Gold (could be gouged into the tree for maximum effect)
b) Coral reefs
c) Product placement in poetry
d) Relevant adverts projected onto the ceilings of post-op recovery rooms, eg for health insurance etc.
e)Safety announcements on planes, and announcements over school intercoms could all have adverts inserted into them
See also organic-advertising-or-foie-gras/