The first day was a Sunday and the air quality was poor. Everything looks dingy in the photos.
The day I left was a sunny day. It had rained the night before and washed away all the smog.
The water cube was where all the swimming events were held. They have turned into a water park for kids and was very reasonably priced. The Birds Nest holds a few events but it is prohibitively expensive for anyone to use it so it mostly sits as a backdrop for photos.