My Aunt Martha always planted flowers in between rows in her garden in West Virginia. These French marigolds are a great choice as they only grow about a foot high, don't require dead-heading and bloom from late-spring to frost. In the background are blooming buckwheat. I scattered some seeds (you can get them at most feed stores) when I planted the spring garden. Kale to the left, beets to the right. Flowering buckwheat attracts not only pollinating insects (such as bees) but beneficial insects (for instance, wasps) that feed on nasty pests like Colorado potato beetles and Japanese beetles. Here's an interesting article from Cornell on the subject.