Since city gardens are usually really short on space, try looking at some Japanese gardens. There are wonderful examples of gardens (especially in Tokyo where space is at a premium) in really small spaces. Many times, these tiny gardens are points of quiet or areas for meditation. One can find all kinds of nooks and crannies in an urban environment to put plants in to create a tiny garden.
Have an ugly brick wall facing you? Put up a wired grid and hang small pots off it. Have an ugly garbage can? Put up a stand with plants in front of it. If you look around your limited space, you can find areas to put plants in. Stoops are good for staggering pots of flowers or herbs. If you have an unsightly chain link fence, put fence boxes on it with hanging plants to hide it. Even the most bare, weed ridden backyard can be enhanced with containers from raised beds to pots and fence boxes. The ideas will come as you start looking around your small space, just be mindful of the watering and sun requirements of the plants.
Balconies can be turned into restful meditation spots with hanging pots, climbing vines and containers. Be sure they are secured just in case it gets windy. As long as the containers have something under them to catch the water there should not be a problem with water dripping downstairs on the neighbors.
A great book on these tiny gardens, which features some incredible gardens on balconies and in backyards, is GARDEN VIEWS, Water and Stream Gardens, by Tatsui Teien Kenkyujo. Another good source is CREATING SMALL GARDENS by Roy Strong. Living in a city, one can miss the natural world of plants and green. But this doesn't mean you can't add some yourself with a little imagination and elbow grease.