What you needBirdseed
Use a funnel and fill a whole balloon with the bird seed. This is a slow process but well worth it.
Cut the neck off the filled balloon. Cut the neck off a second balloon and place it over the filled balloon covering the open hole. Now you should have a completely covered ball with no bird seed popping out.
If you want them to be super strong add another solid balloon with the neck removed.
Take a balloon and cut some holes in it with the tips of your scissors. Remove the neck of the balloon then open it up and place it over the ball.
The colored balloon underneath should be showing through the holes and giving it a nice colorful pattern.
Repeat this with several layers of balloons and holes adding to the effect and also the the strength.
Try to arrange it so the original birdseed hole is always covered by a firm piece of balloon and not a balloon hole.
For a Yo Yo stress ball add a piece of elastic by tying it around one of the holes. Then it can be bounced like a yo yo.