I was chatting with a saleswoman at the San Francisco Apple Store, a few blocks from where WWDC is being held this year. According to her, the top things people wearing WWDC badges bought from the Apple Store are (most popular to least popular):
Laptop Batteries. Specifically, 15″ MacBook Pro batteries. This is apparently #1 by a wide margin. It’s what I was in the store to buy. I don’t normally carry a brick spare battery with me, because in my experience you can’t rely on it being charged when you need it. But at WWDC, there are stations which charge your battery while you work, so you always have a fresh battery ready to swap in. You can go the whole day without plugging in!
Power Adapters. (I guess battery chargers are not a panacea)
Laptop bags/backpacks. This one is funny, because Apple gave out a laptop case to every WWDC attendant. I’ve always thought that giving out laptop carriers to people at a tech conference was a bad idea, because everyone there already has a laptop and a laptop carrier that fits them. Mass-produced swag is probably not going to be better quality then something a nerd picked out for their computer. More people purchased a backpack then a smaller brief-case-sized carrier (presumably so they could haul around extra gear).
So when you go to WWDC, bring an extra battery. It’s the number-one thing attendees (including me) regretted not bringing so much that they ran out to buy one.