Flowers That Plant Themselves
Sure, they're called annuals, but buy their seeds once, and you'll have blooms year after year.
After the Blooming
Once flowers set seed, you can let the plants drop them on the soil and wait to see what comes up where. Or you can collect the seeds and sow them at the proper time according to the seeding procedure. Either way, a small investment of money and time will bring you many happy returns.