How can I download WWDC 2013 videos straight to my iPad?
Not really a way to how to download them directly. In order to get in touch with your iPod, iPhone and iPad using a computer is actually iTunes since iTunes will sync the iOS Software.

This might help : https://discussions.apple.com/thread/4030410?start=30&amp;tstart=0

And some more here : http://www.copytrans.net/support/install-apps-to-iphone-ipad-and-ipod-touch-without-itunes/

http://www.makeuseof.com/tag/6-ways-to-sync-music-to-your-iphone-without-itunes/