I had the 2nd and 3rd graders make a "house." They could make a house for an animal, an astronaut, a fish, a human--whatever. Of course, they mostly went with a space theme, because then they could create space ships to go with the house. :)
The four boys decided to combine and connect their four houses into one epic station that they named the X Illinois Space Control Unit. And yes, it was awesome enough to live up to that name.