Their upturned birdbloc boxes - so versatile! They are just the right size, can go everywhere, no problem with being gnawed on or being shoved around. Turning them sideways I can corner a piggy if necessary.
Also ideal for picking up all the droppings at the end of a busy day roaming the living room, I just turn them up and use them as a basket.
And the plastic kitchen cupboard liner from Ikea to keep accidents from soaking through their mats onto the carpet.