Cut clean cardboard (cereal boxes, shipping boxes) into rectangles of various sizes. In each rectangle, cut a couple of 1-inch slots along the edges. Pile them on the floor.
Slot two pieces together at right angles. They stand up.
A house. Then "more pieces." Then a city. Kids at 4 figure out the slot-into-slot trick in about a minute; at 5, they design.
Pre-cut some windows and doors. Tape the structure to a base for "permanence." Combine with the masking-tape sculpture.