A lot of people don't like kale because it's too stemmy and fibrous, which indeed can be difficult for people to digest who have gastrointestinal sensitives and conditions. A good healthy alternative would be butter lettuce, which is still quite rich in antioxidants.
If you compare the nutritional value of butter lettuce vs. spinach, you will see their ORAC values are almost exactly the same - fresh spinach is only about 6% higher. But what happens when you match a bibb or boston lettuce against kale, which is praised as a superfood? As it turns out, the ORAC difference between them isn't as much as you would - green kale is 24% higher.
In short, if you're not a fan of kale - whether due to taste, texture, or digestive problems - rather than force yourself to endure eating it, the easier solution would be to simply make a slightly larger salad with a lettuce you like such as butterhead. That way, you would still be getting the same antioxidant content. Whichever you choose, all are plentiful in vitamins, minerals, and other phytonutrients.
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010