Think in terms of adapting Japanese styles, based on relationships between plants, stones and water. Japanese gardens are not geometric in form; they have flowing lines and are intended to be meditative. Sand and gravel are used as the surface. Choose a couple of stunning small trees to plant in the gravel. Japanese maples are ideal for city gardens because they do not grow large. They have stunning leaf colour which changes throughout three seasons and in winter they have even more interest because of their spreading shape and delicate branch structure. Japanese gardens aim to recreate the landscape, but do this in terms of large stones representing mountains and gravel swept into waves representing water. Gravel, two little trees, a couple of large rocks well bedded into the gravel, a rake and a bit of contemplation. What better for city life!
A rose garden, on a small scale, is quite possible in a small city space. A square bed in the middle of the garden can be planted with repeat-flowering roses. It is best to plant three bushes of the same rose close together to get a rich effect. For an example of a scented bed, flowering for six months, plant 3 of Graham Thomas (yellow), 3 of Claire Austin (white-pale yellow), and 3 of Shropshire Lass (pale pink-cream). Underplant the roses with geraniums (cranesbill), not the kind grown in window boxes but the hardy outdoor kind, Johnsons Blue, Album, Lili Lovell there are dozens of varieties which will flower from May till September and quickly establish themselves as ground cover. Around the bed have a path of sand and crushed shells on all 4 sides, bordered by a lavender hedge this will grow into a bushy hedge 3 feet high.
A city garden needs to have good drainage, which is one reason for preferring a gravel garden to a patio-style garden. A gravel garden is ideal for a free growing herb garden unusual, different, and bee-friendly. Order your plants from an herb nursery. Plant creeping thyme and chamomile as paths to be walked on. In between the paths plant as many other varieties of thyme as you can find, bee thyme, culinary thyme, white thyme. Plant marjoram, hyssop, lavender, fennel, bergamot, and leave them alone to self-seed, which they will, wildly.