Have you made your own squishy stress ball? You use a balloon (actually several) and fill them with different materials to squish and squeeze. You can fill them with flour, play dough, beans, beads, rice, and many other materials. They are great to use as fidgets, stress balls, or play a guessing game to figure out what is in the ball.
Here is a great video that I found on YouTube which shows a great technique on how to fill the balls and how to close them up. Essentially, you fill a little bottle with your filler material and then put the balloon over the end of the bottle to let the filler poor into the balloon. You then cut off the top of the filled balloon, and an empty balloon and then wrap the empty balloon over the filled balloon to close the opening. You can add multiple more balloons to give stability, and then cut a different colored one to put over the top for a nice design. Watch the video for a great visual on how to make the balls. This would be a great activity for the kids to make with you as well. They can stretch the balloons and measure out the filler into the balloon.