Ooooh the first thing i thought of pink primrose! I love that plant. It fills in so nicely. Blooms all summer. Its one of my favories.

I have my delphiniums interplanted with sages, ornamental grasses, many forms of annuals.

Nepeta works well, but will need shearing back after bloom, leaving mounding lacy foliage. I've not had good luck with the pink varieties here due to humidity, but the pale lilac and blue have performed well and coordinate with pink blooms.

I've not tried the pink hardy geraniums, but my blue-purple 'Rozanne' bloomed almost all summer (when I kept the bunnies away).

Short, pink asters (like Honeysong Pink) for late summer to fall. I've had to stop growing asters due to bunnies.

Erigeron Pink Jewel (I haven't tried it, but thought about it).

Among annuals - pink ageratum, pink angelonia interplanted with a purple foliage plant such as coleus.

Cameron

I think a perennial Geranium would be great. Geranium macrorrhizum is a wonderful plant that can tolerate a bit of dryness if given a bit of shade. G. cantabrigiense 'Biokovo Karmina' is a nice groundcover sort with pink-purple flowers. Geranium sanguineum 'Vision Light Pink' is also great. All of those bloom in spring, but the last one can sometimes rebloom if snipped back. And all of them have nice looking foliage even when not in bloom.

Astrantia (Masterwort) might work if there is enough moisture.

I also think a Platycondon (Balloon Flower) would work well. They bloom in late summer/ early fall so the foliage stays nice all the year until afer it blooms (and even then it turns a nice golden color in fall). I know there is a nice light pink colored one from the 'Astra Series' of balloon flowers.

-Cameron, I am sure you will love the Erigeron. I don't have E. speciosus 'Pink Jewel', but I do have E. speciosus 'Azure Fairy'. I adore it! Looks a bit like an Aster but without the mildew. The only downside to it is that it flops some, so I have mine in the mid-border leaning up against other things.
CMK

excellent suggestion re the astrantia. I have them near my delphiniums as well.

Since delphiniums are tall, of course, so I have them at the back of my bed. So I'm assuming anything I interplant with my delphiniums would need to be somewhat tall to fill in the bare spaces when the delphinium are cut back.

I saw one of the suggestions was balloon flowers. I thought those were very low-growing and close to the ground? I'll have to look up some of the other suggestions as I'm not sure what they all are.

just_me, my white balloon flower gets about 24" tall.If deadheaded seems to keep blooming.
Here is a shot of my white one.
Tammy

Here is a link that might be useful: {{gwi:196580}}

My neighbour grows hardy hibiscus that are slow to show in the spring, but by later in the summer they are quite large and they beautifully disguise some of the less attractive plants she has growing behind them. If you like large flowers this might work for your situation.

Thanks for the ideas, I'm rearranging a lot of my beds this year and will be moving some of my delphs early so need ideas. Gotta get more Erigeron Pink Jewel, I only have one and love it. Maybe I'll seed some. Right now I have Lady's Mantle in front of the delphs in one bed but the fleabane may work better. Might seed some pink baby's breath as well to fill in the space fully.