Here is an idea that I came across in a French town square. I plan to try in my own garden next year. These impressive towers of petunias and geraniums are planted in a series of tubs stacked on top of one another. Once the plants grow bushy they drape down to hide the tub structure.
The largest displays were constructed from seven tubs. The one I plan to try in my small garden was constructed from four tubs. Now I need to find suitably shaped tubs.