As bicycles grow in popularity, so do related opportunities for smart entrepreneurs and marketers. A concept we've increasingly spotted in bike-loving European cities—including on our own rides in Amsterdam—is bike seat advertising.

Ads are printed on plastic bike seat covers, which are then placed over the seats of parked bicycles, keeping seats dry and leaving an unavoidable impression. By selecting specific locations or even certain types of bikes, advertisers can focus on an audience that's as narrow or wide as needed. Parked bikes near universities to reach students, for example, or schools and cargo bikes—the Amsterdam version of the minivan—to target parents.

In Europe, the covers are sold by numerous companies, including Zadelhoesje, which addresses environmental concerns by selling seat covers made of recycled plastic. One to look into if you're in advertising or promo products. Or why not set up a standalone business, offering both printed covers and promotional teams to distribute them?