My boys' favorite hidey houses are step stools with a hand towel draped over them. The towel becomes like a curtain for two of the sides, and they actually love going in and out of the "curtained" sides.
My main herd have wooden houses with a little ramp to a roof platform I put a fleecy pad on and they love those, but the all round favourite with both sets of piggies is always a Rosewood haycube that's been chewed to bits and soaked with pee, they are always upset when the bottom is so soggy it falls off and it has to go in the bin! They like fleecy tunnels too, and seagrass tunnels. Except Tallulah skinny pig who doesnt like seagrass because its grassy and doesnt like fleece because its fleecy. Apart from haycubes and hiding under her handsome furry husboar there's not much she does like in the way of furniture, unless she can eat it loudly while everyone else is sleeping!