Features a model of the H2 rocket and a planetarium as well as robots and undersea vehicles.
There is also the famous organ-playing robot with a TV-camera in his head that was presented during the Expo. This robot absolutely fascinated me at the time and I was quite stunned when I found myself standing in front of it by chance 20 years later.
ACCESS: From Tsukuba Center (Tsukuba Express train, "Tsukuba" station) face north, and look for the big orange rocket in the horizon. That's the place. It's about a 10-min walk from the station.