Get local businesses and organizations to adopt a traffic island or sponsor a sidewalk planter. It could be framed as a citywide contest with a prize. See the link to Peabody Pride in Bloom promotion. Businesses could have a small sign with their name on the island or planter, saying that it was sponsored by them, and that would be very low cost advertisement for them, especially if perennials were planted that had relatively low maintenance costs.
The idea would help promote more visitors to the city by making the area more inviting and colorful. It would be very cheering to the local residents, because who doesn't like to see flowers bloom in the Spring and Summer. And it would be an overall improvement to the community i.e. it could indirectly increase the value of the properties and improve neighborhoods.