Answer: There are so many possibilities that it really depends on what flowers and colors you like! First, determine what the sun exposure is for your garden. I'm assuming that it's full sun, or mostly sun. Some flowers that are easy to start from seed (and that will usually self-sow if you let the flowers dry and go to seed at the end of the season) are sunflowers, tithonia, zinnia, coreposis, cosmos, gaillardia, black-eyed Susan, coneflower, larkspur, aster, and bachelor button.
Hollyhocks are another good choice, but since they are so tall and often look best at the back or against a wall/fence, you might want to sow them specifically. The same goes for smaller ground-huggers such as alyssum and nasturium in the front of the bed, if that applies to your design situation. I hope this gives you some ideas.
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