Cubby House
You could use two chairs pushed apart, drape a sheet, tablecloth or something similar over them and put some heavy on the seat part to stop the fabric sliding off.
When fridges and other large things were packed in large cartons you could open it top & bottom, lie it on its side and make it into a cubby house. We never thought of that but we used to go in it while others went and hid....or simply crawl through it. Those cartons were very strong and you could play in those for months - especially if you kept it undercover.