What was left behind on the moon by the astronauts?

The flag, the science gear, their trash, their footprints, the base of the LM (the gold colored part) and, IIRC, a monument to fallen astronauts, and a piece of the Wright Flyer.

The Apollo 11 flag fell over on liftoff (it was just barely sticking in the ground, later flags were made so the astronauts could drive them in better). After 40 years of lunar conditions, it is safe to say the flags are undoubtedly so much powder. They were modified off-the-shelf Sears flags made of nylon, with a "curtain rod" attached to the top so the flag could be deployed as if it were flying.