Replacement for celery with equivalent nutrients?
Iceberg Lettuce, as it is also very low in vitamins and minerals like celery and has a good crunch.  According to the USDA's "MyPlate", 1 cup of iceberg is 1/2 serving vegetables, and 1 stalk of celery is 1/2 serving vegetables, so to get equivalent nutritional value for 2 stalks would take 2 cups shredded.

You could add more vitamins and minerals by replacing the celery with something more nutrient dense.