The whole question of the Harappan disappearance is the sort that we call "vexed". The basic facts are that around the late third, early second millennium BCE the population of the Indus Valley began to shift from their highly urbanized mode of life to a more dispersed on, and it seems parts of their material culture (eg, certain pottery forms) moved southeast into the subcontinent. It was once believed that this was a result of invasion from the Aryans, Sanskrit speaking people who carried the so-called Vedic culture with them. However, more accurate dating has thoroughly demolished this idea, and even the most generous serious estimates of the age of the Rigveda give its date as several hundred years after the collapse of IVC urbanization. A current popular idea now is to connect it to changes in patterns of the monsoon, but to be perfectly frank "climate change" is basically the ultimate archaeological cop-out, an admission that our knowledge is just not up to the task. Undoubtedly there was a cocktail of social, political, economic and environmental factors that led to the abandonment of cities along the Indus River, and we simply don't have the knowledge to know why. This is a frustratingly common problem is archaeology (Uruk in Mesopotamia and Sanxingdui in China pose similar problems), and cases as well documented as the fall of the Roman Empire in the west are still debated.

The most interesting aspect for me is just how widespread it is. The civilization was dubbed the "Indus Valley" Civilization because, well, it was centered on the Indus Valley, but we have begun to see just how widespread it really was. Harappan influence is seen well into modern Gujarat, and its material appears in Sumerian sites. I am only exaggerating slightly by saying that the Indian Ocean interaction sphere, the great driver of the old global economy, has its roots in this period.

A good source for this is DP Charkrabati's India: An Archaeological History.

Well it had a lot of city-wide plumbing, trash cans we well. But to me the most interesting thing is its language, which we have thousands of examples of, none of which has been long enough to decipher!

Edit: I recall it was composed of thousands of towns scattered over a land the size of France. But I'm just here to throw in the language stuff.