Unfortunately my horses due not care for toys. :(
I have 2 generic "jollyu balls". Just like the Jolly balls but made for kids to bounce on. I found them at yard sales but have seen them for around $10-15 in places like Wal-Mart and K-mart.
You can find the exercise balls for around the same price.
My neighbor has a few pieces of soft flexible drain pipe in her field. Their one gelding will pick it up and toss it around.
Cleaned out detergent bottles are sturdy toys and can be hung from tree limbs and fills with either feed or rocks. You can also place some holes in the jug for feed to fall out of.
We have a stream in our field so it is hard to leave toys out without them finding the stream.
Hide and seek hay piles are good to do also. Give both them and us exercise. :)