You could clean the trash out and plant rosemary, lavender and Nepeta "walker's low" all very drought resistant (assuming the planter has drainage) and low maintenance (no pruning) and people love to brush up against them (fragrant) when walking past. Single stemmed flowers don't usually work, because people steal / cut them. But, yes, it would be ideal if the shop owners or a nearby resident would take ownership, otherwise the planters should be removed, which points to a larger problem of the city investing in infrastructure that it can't or won't maintain; it's worse than useless it becomes a drain on the public realm.

unfortunately the grocery is not a part of the town green special service district. perhaps there's an opportunity to partner there -

LCI removed the three pieces of litter