Cemetery in the sky
It turns out that Sao Paulo wasn't joking back in December with their billboard ban. As the ads have come down, a boneyard of skeletal billboards have been left in their place.
I've always found urban decay beautiful, or I guess specifically I've found it hauntingly beautiful. Walking around the scenes from Henk van Rensbergen's abandoned places photography really puts perspective ("Too much fucking perspective" I can hear David St. Hubbins say) on what we think of as major achievements. Shelley's "Ozymandias" comes to mind.
I think we can agree that the ruins of modern advertising, while perhaps a worthy monument to a substantial victory in reclaiming our own mental environment, are a bit depressing. Fortunately, there are a great number of things with which we can replace these former monstrosities. Some of them might even be useful.
Solar panels are an obvious choice. I've been to Brazil before (they call it "Brasil" there... don't they know how to spell their own country?) and I recall it being quite sunny and smelling like gasoline. I believe the term I used was, "like a wildlife preserve where the zebras drive Hummers". Even putting aside any mock and/or pathological jingoism, solar would seem to be an excellent decision.
Art is another viable option. Why should big multi-national corporations get all the fun? Local communities should get a chance to decide how to decorate their own neighborhood. People are already doing this, just not entirely legally, which leaves a lot of people with important things to say—but who won't jump a fence to say them—silenced.
Finally, call me crazy here but we could just take them down.* Leaving them up as skeletons suggests that they need to be filled, whereas taking them down would leave our next generation with an impression that they never belonged there in the first place. Then, if advertisers ever wanted to put up billboards again it would represent a change in the status quo, both physically and mentally—much more difficult to accomplish. We're never any farther than one generation away from an entirely different world.
There are other things we could do, certainly. Windowbox-style local gardens, windmills, dynamic information (temperature, traffic conditions, etc.), or even painting each one a different bright color could all satisfy the need to turn commerical chaotic into populist pretty. What would you do with your own billboard?
* Actually, this hadn't even occurred to me until I asked someone else what they would do in the situation.