I'm not very experienced but have been successful with runner beans which, mixed with sweet peas, look great as they are growing. You can always buy or make raised beds and fill them with a more nutricous soil if you have trouble with your natural soil.
.....sorry, nutritious! Lettuce and tomatoes also grow quite easily, although you'll need to make sure the toms get lots of feed,water & sunshine and be wary of slugs enjoying the lettuce. Start the toms off indoors & protect early lettuce with cut off plastic bottles. I'm experimenting this year with potatoes in deep buckets from Suttons which wouldn't take up much space in your garden. As you can tell I'm an enthusiastic novice!