Thinking about it carefully, I'd say that wood toys would be the greenest, assuming they're made using wood from sustainable tree farms or scrap wood that would otherwise go to waste. While well-made and properly sealed wood toys can last a person's lifetime with proper care, it'll still break down faster than steel, glass, or silicone. Less of a footprint in the long run.
So basically, anything from Nob Essence or a company that operates similarly.
One day I'll have to try something from them.