Like Joanne, I go to the lumber store and get untreated pine. It's floor moulding, and we discovered that if we get the odd shorter sizes, they can only charge for a full foot, so we often get lots of footage for free. I have a saw, but if I'm not in the mood, I can just step on it and snap it in half to fit in the cage, and it seems to be fine for them. They love these as they like chewing on things they can hold down with their feet, as opposed to smaller things that roll away.
Once every couple months I give them a medium sized box (like the size of a pop can box) to play with. They go nuts for it and chew it up in a couple days. ONce they pee on it, I take it out, but they crawl in it, roll in it, sleep in it and jump on it. they don't constantly have one, because if it's in for too long their poops get smaller, so I take it out after two days or so.
The most important thing I've noticed is to give them differnet things all the time. They even get tired of the pine sticks if I don't rotate them with other chew things, or take them out for a couple weeks so they seem "new". They need variety and new things all the time, so instead of overwhelming them with lots of toys at once, I give them a new one every week. this is more exciting for them and keeps them entertained the best.
To be honest, their favourite toy these days seems to be their bedding
they have never chewed on a lava stone, but it's important to give it a try.