To me a moon garden would consist of white, fragrant, plants like Moonflower (impomea) vine, the fragrant nicotiana, maybe some grasses if you have lots of space like gayfeather, abelia, and so on. I don't have a lot of space so I mostly stick to the moonflower vine with its heavenly scent (blooms late in day and stays till morning) bacopia which spreads nicely and is problem-free.