In 4th grade we've been studying the geology of planets/moons/asteroids, and used what we learned to create our own planetary bodies with oil pastels. 

Next up we are designing our own mission to mars. NASA's Jet Propulsion Laboratory developed a card game that requires the kids to choose their own rockets/communication devices/machinery/scientific tools, keeping in mind cost ($250 million) / weight / power constraints. You can play the game yourself at home using this link: https://www.jpl.nasa.gov/edu/teach/activity/marsbound/.