I think the wearable tinkering community has plenty of ideas for input devices to draw from. Such as this little one-handed keyboard: http://chordite.com/ . A touch panel (doing the same job as the one on the side of Glass, so you could operate Glass without reaching up to the side of your head) could be located pretty much anywhere and blend in quite well, on a belt or watch or some such. Someone else mentioned Myo (https://getmyo.com/ ); that would be nice too.

As long as the interface makes the number of choices available at each "page" of the interface pretty low, then input devices get small and easy to hide.