I think those are both good suggestions. It depends on what grows in your area, how your soil is, how much sun you have, and so on. I like the peonies planted in with a jumble of other plants. They don't appear to mind being interplanted with spring bulbs, with shrubs close by (box and old roses are some of my favorites), and various small annuals making their contribution. Irises nearby (not on top) are a good foliage contrast. I have a stiff clay soil and dry summers so don't have to worry about air circulation, and it's so sunny here in summer that I can plant my peonies in part shade and they do well. I plant for succession of bloom and contrast of plant habit and foliage texture.

My peonies and Iris are blooming together about the same height with a good color contrast. Al

Are you looking for things that bloom at the same time or something that blooms later to look good with the peony foliage? False indigo blooms at the same time as most mid blooming peonies as does mock orange and the irises mentioned above, all look pretty combined. Roses, daylilies, lilies, candy and blackberry lilies look good with the foliage later. There are some really pretty pictures of mixed blooms in last spring's posts, page back and look at some of those for ideas. Think both Jackie_0 and Illinoisdoglover posted some beautiful combo pics. I'm sure there were others as well but those are the ones I remember most!

I have some Iris planted with mine(although I only have one of Iris blooming this Spring), Knockout Roses, Russian Sage, and Autumn Joy Sedum. I also have what I think are Rose of Sharon Hibiscus, but I'm probably going to remove those in the fall and give my Knockout Roses more room to spread as they will probably need it...Most of my Peonies are pretty immature along with the Roses and Russian Sage, so there is a ton of open space in between plants right now. I'm anticipating that to change in the future as they all grow over time. I plan on adding more Iris in between the Peonies eventually, but that's assuming I can ever get them to bloom. lol

I have catmint, irises and daisies planted with mine. they don't seem to mind! :-)