This is an outstanding natural history museum, with beautiful, curiosity-inspiring exhibits exploring the animal and plant kingdoms and the prehistoric world. It's also a real bargain - general admission to the vast permanent exhibition areas is just Y600. For high school students and younger, and adults 65 and older, it's free.
I would say this is a nice place for scholars and grown-ups. If you're around with kids, you'd better not miss the ueno zoo tour!