Certified GKH Gardening Expert
I would suggest bulb flowers that compliment the peonies, such as miniature daffodils. It's also fine to partner some perennials with your peony, depending upon the size of your pot and the needs of the plant. Late bloomers like small chrysanthemums or yarrow will add color. Just make sure your companion plants don't take up the nutrients needed by your peony.
Here's an article all about growing peonies in containers: