Next year, I want to plant perennials (or annuals) that bloom all summer, or nearly so. I live in Zone 6. Any suggestions? I like cool colors, but might add some warm ones for balance. Thank you all!

Coreopsis 'Autumn Blush' "Tickseed" - peachy-yellow flowers with red eyes akk summer. During spring and fall the flowers take on a wine rose blush. Grows up to 24" and thrives in zones 4-9.

Coreopsis r. 'American Dream' "Tickseed" - Thread like foliage with small delicate rose-pink flowers each with a yellow center all summer. May be somewhat spreading. Grows up to 10" and thrives in zones 3-9.

Coreopsis r. 'Heaven's Gate'
"Tickseed" Same as above.

You might also look into roses that bloom all summer.

This year was the first with a semi-stable garden for me. I had lilies pretty much all through the summer (the herbaceous perennial kind, not the bulb kind) pretty much all summer, and black-eyed Susans toward the end into fall. My roses bloomed all summer, too, and would still be blooming if I hadn't cut them back for the snow that came last night.
You can check out the Missouri Botanical Garden search page and select your zone and desired bloom time (as well as a lot of other options) to come up with some ideas.

Thank you! DAG