Quite often you'll see bags of play balls go on sale at retailers or even up for grabs on Craigslist. We have used LePage's 5 minute epoxy before with great success when gluing together playballs (we used to work at one of those giant habitat trails for children back in our younger years... and had some time on our hands). Please make sure you use it outdoors and it's probably best to keep your children inside during glue time.
When the balls are grouped together they are surprisingly strong and stable. If you have time a light sanding with a 120grit sand paper on the surfaces to be glued could be beneficial, although we never did.
We love the idea of using them in simple ways, such as the coat rack on the wall in the photo above. We can also see them being used as feet for an end table! Keep your eye out this summer at garage sales and see what you can come up with, there's many ideas just waiting to be had!