My garden is far from perfect, but along the street are small zinnias that have volunteered for several years now. We see fritillaries, swallowtails, small white butterflies, bumblebees and abundant skippers in them. They don't seem fazed by pedestrians and dogs walking by.

They started with a seed package from the big gardening store, but I don't remember the name. Look for multicolored zinnias with flowers about 1 3/4 (5 cm) across. In the fall I collect dry seeds and scatter them about to encourage the next year's growth.