Bachelor Buttons come back from seed. So do Cleome, bus less reliably, and though i originally planted pink, white and purple, only the purple comes back. Snapdragons, i find them in the cracks of my concrete, and they thrive there. Moss roses, if you let them go to seed instead of picking off the dead flowers, they will come back like crazy, but never the same color. Flax comes back. The best easiest is columbine, a perennial, that reseeds reliably. Hollyhock reseed, and for the first time, this year my red sage reseeded from last years planting of an .89 plant.

Thanks for the reply!
Does it matter what variety of snapdragons and sage?

I didn't even know there was a difference in snapdragons until i realized mine were about 3 feet tall, and my neighbors always stayed so small and perfect in her border. For awhile I had tall colorful spikes poking up in the worst places, like a bad haircut. I yanked them all one spring and replanted the Chimes series, that only grow 8 inches tall. I don't mind them in odd places now because they just tuck themselves in.
I plant salvia greggi, 'Furman's Red'. I have sages that are perennials, but this usually dies for me. I love it so much I save the seeds and scatter them. Hummingbirds love it.

Thanks for the update! I'll have to start browsing the seed catalogs!

There is Cosmos sulphureus, so much so as to be considered invasive maybe. And do not forget Morning Glories. Grandpa Ott might be invasive too.