We're so used to seeing bees and bumblebees hanging out inside flowers, wouldn't it be fun for once to have some flowers hanging out on a bumblebee's back? Pop a 4-inch pot inside the container and swap out types and colors as the season progresses - calibrachoas, lobelia, or bacopa are just a few ideas. Or, plant directly into the container - succulents would be a great option. Have fun whatever you decide to do - that's part of the joy of gardening.