Take turns being the "constructor" and the "instructor". The constructor and the instructor sit on opposite sides of the display so that neither can see what the other one is doing.

The instructor builds something using the duplos in the bag. Then the instructor gives the constructor directions to build a duplicate. The instructor should give very clear directions and try to use as many mathematical words as possible, such as "perpendicular", "parallel", etc. The colors in the structures should match, too. Don't peek!

The instructions might be something like:

"Place a long yellow duplo with 8 pegs on top of and perpendicular to a long blue duplo with 8 pegs so that the center of the yellow duplo is on top of the center of the blue duplo."