The basis is cardboard and I decorated it with orange and green paper, as well as red and yellow craft foam sheets ("dekorgumi"). The door and windows are laminated and glued on the surface. The sky is made from coloured paper and the green grass is painted.
(E. has already torn the clouds partly, so I strengthen them with Cellotape). I hide animals, people, babies, pieces of furniture etc. behind the flaps. Whatever works.
It's a good idea to stick the house on the floor or on the wall as while the kids are fidgeting with it, it moves a lot and makes it more difficult to open the flaps and easier to tear off a window.
E.'s favourites are the sun and the butterfly. She picks them and walks around with them in her hands. When I changed the pictures she was surprised at the change. Now she's found the box of pictures and not surpised any more. She tries to change the pictures herself.