Gerbils love tunnels and underground dens. They shouldn't be given plastic as they'll chew it and there's a risk of them ingesting it. They love things that they can shred and make into nests too, like the seagrass products. Cardboard tubes are a hit too. Also, you can often find big loofah pieces in the beauty sections of places like Wilkos, we cut those into smaller pieces and they're a much cheaper version of what pet shops sell