It's even better if the area you choose incorporates an area of garden with shrubs and small trees, so the cat has a ready-made jungle to investigate. If not, you can have some fun planting things that appeal to cats, like bamboos that rustle their leaves enticingly in the breeze, and loose ground cover plants under which the cat can hide. Some shelves could also be attached in various parts of the run to provide places for sunbathing, but be careful to ensure that there are some shady spots too. This is especially important if you own a white cat, which may be vulnerable to the sun. Litter trays should also be provided, or your cat will use the flower beds.
Hooded trays are ideal and can be hidden amongst the shrubs.
Another idea is to enclose an area with something like log roll and fill it with sharp sand. This can then be sifted through and scooped much the same as you would with a litter tray. Wide shallow terracotta pots from garden centres can also be used, fillled with grit, and they won't look out of place in the garden setting. The debate on indoor cats will run and run, but, as you can see, it is possible to protect your cat and give him or her a stimulating environment.