Apple is great!Some hamsters also like willow,and I've seen things made out of birch as well if you can get that.As far as any wood goes,just make sure there are no chemicals or pesticides on it and sterilize it properly just to be safe.
Wood really depends on the individual hamster.Some hamsters don't even like chewing on wood,and others merely enjoy stripping the bark off the wood which is at the least good for boredom breaking!If you can test various woods first or make prototype toys,that is a great option and can help a lot in terms of making sure they'll use the wood or the specific toy design
