Let me think... my buns love a carboard box stuffed with newspaper strips, hay and with treats hidden in it so they can dig.
They also love fresh twigs, especially olive! If you can get them a treat ball to fill with small pellets, it's a beloved toy that will amuse them to no end. Some people in here will sure explain you how to make treat balls out of toilet rolls, it's very simple and totally inexpensive. Maia also loves those plastic keys that you give children to cut their first teeth on, she throws them around and chews them and those ones are pretty inexpensive too. My buns also love tunnels and hidey holed but if you make them out of cardboard they tend to chew them, which could cause blockages if they swallow.