My school director mentioned to me once the idea of filling rubber gloves with hand sanitizer as kind of sensory bag. I liked this idea, but I didn't have rubber gloves OR enough hand sanitizer, so once again I had to improvise. I did have play dough, and balloons. So why not use those?
Of course, I was improvising so I didn't actually have a funnel on hand, which left the question of how to actually get the play dough IN the balloon. My first attempt was to do it by just shoving the dough into the balloon, which worked to an extent. It left play dough in the mouth though, and was messier. Eventually, I remembered an old scotch tape roll I'd saved, thinking it might come in handy. Turns out I was right ! It opened the mouth of the balloon enough so I could stuff it with play dough.
After that, just tie the balloon tightly, and voila! Balloon fine motor muscle stretcher!