Pour all-purpose flour for baking into your balloon for your stress ball. This creates a ball that is soft and squishy and will more or less hold the shape you form it into.  You can also use other common powdered baking ingredients that you have on hand, like cornstarch or baking soda. Or use sand, if you have it available, for a slightly grainier texture. Note that this filling can be messier if spills occur, but it is popular for its low expense and the satisfying texture it creates for a stress ball. Try small dry beans, lentils, rice, or birdseed to fill your balloon for the stress ball. This creates a firmer, more textured ball, like a bean bag.  This type of filling doesn’t create as squishy of a ball, but it can be easier to get into the balloon and creates less of a mess if spilled. Note that birdseed or dry goods may have sharp or cracked fragments that could create a hole in the balloon for your stress ball. Prevent this by using several layers of balloons to make a thicker surface for your stress ball, or opting for a smoother filling material instead. You can also mix dry beans with flour for a filling that is a mixture of firm and soft. Try half beans and half flour, or whatever combination creates a texture you like. Use a modeling clay product or playdough to put into your balloon. This creates a stress ball that is very squishy and holds its shape well.  Be aware that many modeling clays and play doughs will dry up over time when left exposed to the air. Though you will be putting it inside a tied balloon, it may not be completely airtight and your stress ball could harden after a couple days or weeks. Note that solids like modeling clay or playdough can be more difficult to get into a balloon successfully. Use a funnel as with any other filling, and roll the clay or dough into thin snakes to help get it through and into the balloon.

Summary:
Use flour for a soft and malleable ball. Use dried beans or birdseed for a firmer ball. Try modeling clay or playdough.