The celerys is primarily for crunch and to give a contrast in texture. So, you could use radishes as another reviewer suggested, chopped water chestnuts, bell pepper, or jicama. Try replacing the green onions with thinly sliced shallots, or to replace them with a different herb try fresh dill or tarragon. It’s a pretty versatile base recipe, so let us know if you come up with any fun combos!