If you want minimal effort, you'll probably want perennials (means they come back year after year - although that doesn't mean indefinitely).
Most plants flower once at certain times of year. So I'd advise from now until October time, visiting garden centres maybe one a month and looking at perennials that are flowering (or about to) at the current time. Picking what you want and planting it. That way over the year you will have something flowering at all times. And next year you won't have to do much except weeding. And you can feed stuff if you want. Rose bushes are nice, you can get ones that keep flowering from June/July until the first frosts.