We are in the energy module, and I would love to see a simple "build an energy efficient house" app. We've talked about all sorts of choices that would help a home be more energy efficient. We could have groups of choices for the house: studs, SIPS, or other structural choices; shapes that can be made bigger or smaller to serve as windows and doors; different R values of insulation; different types of heating/cooling, etc.
The students could make choices about the different characters of their home, and two values would be summed: 1) an energy efficiency and 2) a total cost to build the home. Students would then have to make choices about whether or not the building decisions they make also make economic sense.