When I was a kid, I loved toy bricks. (Think "Lego" and it's knockoffs.) I'm thinking of a construction prototype tool, consisting of a scaled-up version. They can quickly be arranged, attached to each other, and shaped into the end-shape of the building (which will be kind of weak due to the light weight construction, and the fact that they're hollow), and then modified to provide the forms to fill with a better construction material, like cement. (The modification will probably consist of drilling a hole in the side.)
Hopefully it will also come in multiple sizes, to allow non-rectangular forms of architecture as well. Also, window kits are essential. Same for doors and other common features.