Could you try whimzees, not sure where you live. Or something which is cardboard covered in hay - something like this
https://smile.amazon.co.uk/Rosewood-...gateway&sr=8-7
or something like this.
https://smile.amazon.co.uk/gp/produc...?ie=UTF8&psc=1
Also walnuts still in their shell, if your hamster doesn't chew it, try starting off a little hole in the top of it with a cork screw or something similar