Or, somewhat less messy and involved:

Take a balloon. Blow it up and deflate it once to stretch it out. Stick a funnel in the end of the balloon and pour poppy seeds in. (or rice or any grain - the smaller the better it feels on your fingers, but then larger grains sound a bit like you're breaking someone's neck, so I guess it all depends on what kind of stress you're looking to relieve.) You might have to push a bit, but they're discrete enough and heavy enough that this isn't a serious problem. When balloon is full enough for your taste, tie it off. Tada!

If you want a perfectly round one, you need three balloons, but it's even easier. Cut the ends off of the balloons. Fill one with poppy seeds or the grain of your choice. Pull one of the other balloons over the opening (try to keep air out), so that the bottom of the second balloon covers the opening of the first, filled balloon. Then pull the third balloon over the opening of the second, as a final seal and fail-safe. Tada!