There are some images and videos of the building here:
Given that it's protected by security, it can't be something trivial like a sound stage or an after-show party area. They probably won't ship actual Apple products until very close to the event but the delivery people and workers sign non-disclosure agreements. The building looks wide but narrow and there are two smaller areas sticking out the back:
That looks like the entrance there and you can see it doesn't look like it goes back very far. The comments in the above article say:
""It looked like a sound stage going up for a rock concert behind Flint Center," Mangini said in describing the scene two Sundays ago, with lights on a grid above the stage being built in view of a nearby fountain and duck pond.
Mangini said the structure had become even more enormous by last Sunday's walk, with walls around the stage structure that he estimated at three stories high. Mangini, who was an electrical engineer and now teaches part time at UC Santa Cruz's Silicon Valley extension campus in Santa Clara, took note of huge power cables run through pipes to a transformer at least a couple hundred yards away.
Mangini asked a worker about the construction project and received a terse answer.
"We really can't say," the worker told him. "All I can say is this is one of the largest shows I've ever worked on, and I've worked on them for 30 years."
Mangini, who moved to Cupertino in 1974 and celebrated his 50th anniversary with his wife last year, said the construction and huge amounts of electricity piped to it seems to point to some kind of light show or rock concert -- or both -- happening outside Flint Center.
"It's a very professional job," he said Thursday. "It looks like Hollywood to me -- there's a lot of money going into this.""
It doesn't sound like it's going to be something lame like a Homekit demo, construction workers must put up demos like those all the time. When they spoke about Homekit, they mentioned quite a lot of controls and partners:
People don't walk around their houses with their phones so it would need a wearable to control it all. A front door can unlock as you approach it while wearing the device and notify to ask to lock it if you walk away from it. A garage door can notify to ask to open as you drive a car into the driveway. You'd be able to change lighting in different areas, adjust heating and so on.
But, like I say, this sort of thing would be lame because it's not practical for average people. This is the sort of thing Jony Ive would put in his $17m house.
The demo could be extended to use the wearable for shopping, fitness and other things but that's still not as groundbreaking as the original Mac, iPod or iPhone.