Structual Elements
A pergola, perhaps incorporating trelliswork, and perhaps in a more interesting colour than usual.
A summerhouse or covered seat, or maybe a pavilion at the end of a vista.
For smaller gardens, how about positioning some urns or pots in the flowerbeds that you can keep planted with seasonal plants?
Stone statues, urns and similar items make marvellous features? It often looks best to have more than one, so that they don't look 'plonked' down.
Sculpture looks at home in a garden when well-positioned? Maybe there is a local artist you can support?
Automatic irrigation is worth considering, as plants always grow so much better with it, and it is an efficient use of a precious resource, as it delivers water to exactly where it is needed.
Lighting means that the garden becomes a feature at night as well, and is great for those Summer barbecues and parties. There is a choice of mains or low-voltage systems, both of which have their advantages in different circumstances.
Water is always a fascinating feature in a garden, whether it is a waterfall in a rockery, perhaps a meandering stream, a formal fountain, or just a wall-mounted feature on a terrace to give a soothing sound. Can you fit it into the landscape naturally, or will it look out of place.
Maybe you do not have time to do a great deal of gardening, so consider asking your designer/landscaper to come up with a low-maintenance garden; it is easily done, and can look just a good as any other.
Do you want a shed? Maybe not, but perhaps you need one!
What about a greenhouse, however small? Propagating your own plants is one of the great joys of gardening.
Don't forget the front garden --you will probably see it many times a day all year, so both the design and the selection of plants needs very careful consideration to make sure that it cheers you up whenever you pass through it.
If all this sounds far too complicated and far too much to commit yourself to, don't worry! A garden can be made up from the simplest and most economical elements, and a professional landscaper/designer will be able to guide you through all the options to provide you with the garden you want at a suitable cost. Then, once it is all completed, you can look forward to many years of enjoyment and relaxation, and an appreciating asset.