To make this simple and healthy treat, start by washing and chopping fresh celery into bite-sized pieces. Celery is low in calories and packed with vitamins, minerals, and fiber that can aid digestion. Next, cut up some juicy watermelon into small, seedless chunks. Watermelon is a delicious source of hydration for dogs and contains beneficial antioxidants. Combine the celery and watermelon in a blender and blend them together. You can serve it as is or freeze it for a while to make it extra refreshing.