My garden design is casual enough that I can accept self sown plants. They always thrive where they put themselves; although it is often where I don't want a plant at all Many come up in the lawn or path. Foxgloves are one of my favorites. I plant one packet of seeds about once in five years to keep the color mix, other wise they all come up purpley/pink. The first season you get a rosette of leaves, and with some varieties, flowers. It's the second year that the flowers are spectacular. Then you let them go to seed and plant themselves where they want to be, or try to influence the outcome by shaking the seeds where you want next year's plants. My garden is on a slope so gravity has a say. Some years all my volunteers are at the bottom of the hill. Some other favorites are Lunaria (money plant) which I grow mainly for the handsome purple flowers in the spring. There are always too many so I pull half of them out when the blossoms fade and let the rest make "money". Another good one is Sweet William (Dianthus barbatus) which flowers spectacularly for me but never self sows. Butterfly weed, a perennial, acts like a biennial in my garden and pops up in places that I never would have planted it. This year it made some spectacular choices. Also the color variations are interesting, ranging from almost red, through orange to yellow. All these plants are Biennial but you end up with flowers every year because the first season of growth starts when the seed hits the ground (this year) and next year you get the flowers.