Raw marrow bones, a piece of really thick rope (not the rope thingys you get in the pet shop, they are made of cotton and won't last) a thick pruning from a fruit tree. As said a plastic bottle, lid off, with a bit of kibble inside.
Having said all this, we don't own a stick of furniture or a best shoe that is without puppy teeth marks or not well and truly gnawed. Like all babyhood it doesn't last long, so enjoy it while it's there