If you want something greeny and low maintenance instead of the mulch around the flagstones, could use something like Gypsophila repens Rosea (Pink Creeping Babys Breath), Silene acaulis (Moss Campion), Thymus Albiflorus (White Creeping Thyme), or Mazus reptans Alba. All are low-growing, grow in Zone 5, are supposed to tolerate being stepped on two or three times a day, and don't require a lot of water.
