Another place that could be cool for an advertisement: on the belly of an airplane! Yes, not many people may see it when up in the air 6km, but when close to the ground, you would get your target audience and some. Would be especially effective for planes that fly over islands where everyone is at the beach.
Caution: these ads should be a tad bit more creative than a phone number.. imagine.. use the odd shape of your canvas.. I think it would be a good place to put ads for toys, movies, entertainment 🙂