Bicycle powered iPhone charger

Hello, I recently built-up a front wheel for my bicycle that has a 6v/

3.5w dynamo. These are designed to power headlamps, but I'd prefer to store the electricity in a battery which could charge my phone. I believe iPhones charge on standard USB.

My first question is about the battery. Of course, any cell or chemistry would do, but I'd like to use NiMH AA cells so that I could easily replace them, and use them in other devices like cameras and radios while I'm bike-camping. From what I've read, charging NiMH isn't easy, so I'd prefer to find something off the shelf to use. Is it possible (or advisable) to use a AA charger as a battery while also charging the cells? I'm quite in the dark on this topic, and would appreciate any guidance I can get.

Next, once I have a battery pack that's being charged by my front wheel, how should I go about charging the iPhone. I will likely use an off-the-shelf iPhone car charger which are often little more than cigarette lighter to USB adapters, but these are made for 12v, so I assume their circuits will be mostly useless to me. What would be involved in creating a USB outlet on my battery pack?

Once those two fundamental questions are answered I'd like to dream a little; It'd be most awesome to have a loud speaker built into the battery pack so that the iPhone could also bump tunes when I'm riding with friends. Assuming I can somehow re-purpose an OTS iPhone dock, I should have access to some audio wires, and run them to a volume dimmer and then to a small speaker. A headlight would be handy too, but I'm not sure that there will be enough juice for all that. Just wanted to throw that out there so y'all will know where I'm going.

And just to put minds at ease, I'll use a good sturdy protector case to house it while it's mounted on the bike. I don't want it to get hurt in an accident, but it'll be handy to have maps available on my handlebars.

This is my first serious electronics project, so I'm all ears.

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