anything they're NOT allowed. I've bought them hundreds of toys since I got them last year and they've shown precisely zero interest in any of them. I've tried balls, twigs, wooden and grass houses, cardboard boxes (which they chew for 10 mins), willow balls, grass balls, even bath mats for them to sit (and pee) on...nothing!
Of course the landlady's rug, the corners of the wall in the fireplace (the WALL!?!?!?), the radiator, the doors, the door frames, any cable tehy can get their teeth around (we are on our 4th replacement laptop charger, 6th phone charger), any bit of carpet they can tug up...
Figures dunnit