We've done this before, but we did it again and they loved it.  I made a race track out of duck tape down the hall and through the living room.  This time Kael had the idea to make mountains so we grabbed a bunch of pillows and stuck them under the tape in various locations.  It was a blast.   Later that day when we were outside and he could see the buttes out in the distance Kael noticed that they looked like out race track mountains!