You could stick chopsticks in the holes and make a chewy hedgehog.
You could poke food treats through and/or into the holes and make a foraging toy (peanut, almond, dried fruit, carrot stix etc).
You could use a very large crochet hook and pull loops of shredded paper through the holes and make a foot toy.
You could do the same thing with (clean) grass or shredded lettuce, kale or other green leafy veg.
You could string a few of the balls onto coathanger wire or paulie rope and fix the ends so the toy hangs horizontally. Birdies can move the balls back and forth along the wire/thread.
You could hang them vertically instead to make a peck-toy.
You could get some long, skinny SS bolts, poke one right through each ball and add a nut (preferably a wing-nut - not the bird, the hardware! LOL!) at the other end. Birds like wiggling nuts off.

This could be a foot toy, or you could hang it so the bolt is opposite where birdie sits and he can worry at it to his heart's content. If he manages to winkle the nuts off, use a magnet to find them at the bottom of the cage (my eyesight is not that good - it takes me a while to find them just by looking).
PS. If you make a foraging toy, hang the ball by a string or bit of paulie rope from the perch. It doesn't take long for a bird to learn how to 'fish' the ball up to his foot, where he has the job of holding it while he winkles the food treats out.
PPS. I haven't tried this yet, but here's another idea. Drop the whiffle balls in boiling water and let them heat up for a few minutes. When they're soft and pliable, see if you can't force two or three pony beads in through one of the holes: I'm sure you can get 5mm pony beads. When the balls cool down, the beads should be firmly (safely) stuck inside and make a jingly toy to hang or to use as a foot toy.
Hope this helps?