Summarize:

You can create an alternative outdoor litter box for cats by using a new or old sandbox that is at least as big as a large litter box. Fill it with soft, fine-grained sand (this can be bought by the bag at hardware and home supply stores), which cats find especially attractive. With luck, cats will be drawn to the sandbox instead of your garden and use it much as they would a litterbox.  Make sure to scoop the sandbox regularly; if it gets too full, cats may return to using your yard. Keep children away from the cat sandbox, and make sure they understand not to play in it. If you want to keep cats out of your garden, but don’t mind them being in other areas of your yard, then try making a cat-friendly area. Cats will be attracted to areas covered by certain plants, and hopefully leave the rest of your yard alone. Create a small plot and grow one or more of the following non-toxic plants in it:  Catnip (Nepeta cataria) Catmint (Nepeta mussinii) Cat thyme (Teucrium marum) Valerian (Valeriana officinalis) Spider-plant (Chlorophytum comosum) If you have a bird feeder near your garden, move it to another area of your yard that is further away. Ideally, you should hang the bird feeder in an area that is too high for a cat to jump to. Otherwise, a bird that visits for a meal might be in more danger than your garden ever was.
Make a cat sandbox. Attract cats to a cat garden. Move your bird feeder.