Perennials are fine in their place, but you might want to mix in some annuals, too. Look at them and read the description. You are buying what you are seeing, except for ultimate size. When they pass out of bloom, rip them out. They will re-seed. Pay extra for more expensive annuals which almost always means they are harder to grow from seed which means they re-seed less than the type. Your time in clean-up is worth something, and this is where you buy time.
Perennials will bloom for some period, maybe a month, plus or minus. Annuals will bloom for longer, maybe up to two or three months. Your Dahlias have to be collected and stored dry and semi-dark. I'm a heathen so I don't know much about Zen gardens, only that they're supposed to be simple and contemplative, right? I have a semi-controlled jungle, an American version of A Secret Garden which may be the antithesis of Zen. I'll just speak to perennial varieties that are tough. Early means May-June, late means July-August. There is overlap depending upon local climate.
Tall (1 1/2 to 3 feet): Early; Virginia Bluebells, Columbine, Foxglove, Lupine. Late; Delphennium, Oriental Lilies, Tall Phlox, Crocosmia, Conefowers. Low, including groundcovers (>1 foot): Ajuga, Phlox, Periwinkle, Sedum, Thyme. For foliage, look at Hosta and Huechera which come in every size and many leaf colors.
Deadheading: All plants flower only for the purposes of propagation. If you prevent the flowers from setting seed, they will continue to bloom for a much longer time. Also, if you prevent them from setting seed at all the plants will live for more years and be bigger. Plants that are allowed to go to seed will seed your space randomly and that means weed-like plants growing wherever the wind blows them. You want them in their place, not all over the place. All perennials that are happy will expand their clumps and need to be divided every few years. Generally, cut them into quarters, give away 3 and replant one. Most of the above are available in a range of colors within their own particular range of colors. Anything low priced and listed as, "Easy to grow" means it's a weed, stay away from that. Pay extra money for named varieties. Buy dwarfs rather than non-dwarf. Dwarf means it grows smaller and slower than the type. It does not mean it stays small. You don't always get what you pay for, but you always pay for what you get. Better to buy fewer plants of higher caliber.