The cardboard tubes full of hay are a good hit for Attila. The girls like the treat balls I make - cardboard tubes cute into ringlets, slid over each other to make a ball. Right before the last ring to cover the last bit of space, I toss in raisins or whatever. They especially like playing Keep Away with each other. Pretty much free to make, too! Other things are treat puzzles (you can buy them for dogs or cats), food balls, and of course you can be creative and try hanging various toys with twine or some other plain string from the top of their cage for them to bite and chew on.
For boxes, if you need to get them from somewhere, go early morning to most department stores. My work, Homesense (Homegoods in the US; Winners, Marshalls, etc) gets a truck in the morning and we usually have lots of boxes. Ask politely and, at least in my store, we'll do our best to hook y'up. The boxes tend to have minimal ink, and the buns love them.