You would be best checking with Stadium direct or with the concierge in your hotel as the will have the latest information, hope this helps

The air quality should be ok. I wouldn't bother with the tour. Just pay at the entrance and you can make your own way up to the roof. There might not be a lot to see though.

There are some great Olympic Games souvenirs inside the stadium. A “Normal” entrance ticket is 40 Yuan (20 Yuan for concessions). I recommend the encapsulated section of 2008 Games grass/soil, price 150 Yuan.

Green Line subway (#8) is best, getting off at "Olympic Sports Ctr". Green line intersects directly with Purple Line (#5), Light Blue Line (#10), Dark Blue Line (#2), Yellow Line (#13) and Changping (light lilac line but no # yet). All other subway lines require 2 transfers. So where you board depends on where you are starting from, but you want to end up on the Green Line (#8). There are color coded route maps in all stations and also most subway cars. Also each subway car has an electronic sign that notes all stops on its route, and highlights the current one. Signs are in both Chinese and English, as are oral announcements. You can find a great subway system map by googling "Beijing Subway System", but I can't reproduce the link here. Have fun!

As mentioned by others their is no reservation needed. Just pay entrance fees at the Birds Nest. About $5 - $8 USD for Cube and Birds Nest respectively. Worth the visit. Beer Garden was opened near the Cube back in summer months so might be in March. Enjoy.

I was there in May and weekdays are best for sure. It does get busy lunchtime onwards I would say. Mornings definitely better for pics although I got some good shots at night, there's lots going on. Enjoy