A shoe box is the simplest - a box you cut holes in! I know some people have made little cardboard compartments to make rooms as well - but then you need a fairly large shoe box (and not sure how the compartments would fit).
Just a nice coloured cardboard labyrinth house would be great! Cheap to buy and replace (although then it couldn't be plastikoted to make it wipe clean).
Going back to plastikoting inside houses - I know the germans didn't used to believe in any kind of waterproofing paint on the rodipet houses as they thought it would stop the wood breathing (which it probably does) and they used to stick little ceramic tiles inside the houses in the toilet compartment
Like small mosaic ones. Which I thought was a lovely idea - if fiddly. A tiled bathroom lol.