What about a saddle bag? With the Mac book air, and the rest got I would think a Carradice Nelson or a Cadet would work great. There are other manufacturers as well.

http://www.carradice.co.uk/

I use the Arkel commuter, you cinch it down to when you don't have much in it. And it has a great suspended laptop holder. I carry about the same amount of stuff as you with a macbook instead. The Arkel is 2 years old and it looks brand new. I ride with it year round. They're expensive, but it works perfectly and it's commercial grade tough.