Cordylines do well in pots - get the biggest you can - tall strappy evergreen leaves, and tough as old boots. Bamboos do well in pots and give height - plus, as they are thugs in the garden, it's best to confine them in pots! Geraniums are easy - lots of colour and easy to find - and you can often get them to go through a mild winter too. If you want to try your hand at something edible try a few raspberry canes. Four in a large square pot would work. Very forgiving of beginner gardeners and they come back every year. Growing in pots lets you try out plants you might not be able to grow in your normal soil - blueberries in ericacious soil for example, azaleas, pieris. You could try a few seeds in pots too - nigella, cosmos, and snapdragons come to mind. Good luck and enjoy!