Excersise balls are NOT safe for guinea pigs. I know tha tcompanies that make these market them to guinea pig owners but these are dangerous for them and cause serious back, feet and toe injuries. There are many other great toys for guinea pigs, this site has great information on toys for guinea pigs and ways to allow them to get exercise, as well as great tips on general care.
http://cavycages.com/toys.htm
Hope this helps and if I've answered your question don't forget to please accept my answer and to make it twice as nice a bonus (no matter the size) is always appreciated!
Thanks,
Rebekah