The past few weeks I've been debating putting in more prairie-esque plants in my garden, and recently finally decided to make the addition. In the end there aren't very many prairie-like plants, but I decided with limited space I would rather keep the plants well delineated and visible than try to make a 20 square foot grassland.
The border for the garden is natural river rocks, which I continued with in the expansion. Rocks don't have great weed control because there are always cracks between them that weeds will creep through, but on the plus side they are large enough that weeds don't seem to grow under them very often. Additionally, larger boulders can be put in among the flowers as accent pieces. These help keep the garden looking nice in the spring and fall when the flowers are still growing or starting to die back.
Buying flowers this late in the season also means most things were very discounted!
Taller plants went towards the center of the garden (the tallest being prairie grasses and a rose bush that was already there), and then tapered down to the shortest plants around the edges of the garden. This ensures you can see lots of plants at once when looking at the garden, and also makes sure that the plants do not block each others light.