How to convert 6V~3w input to 5V 500mA output (USB)

So I have a slightly strange project, and I'm a total beginner with this stuff, so bear with me if I'm asking stupid questions! I have a power source that generates 6V~3W, but that power varies because it doesn't generate constantly (it's a hub on a bicycle, so the faster you go, the more it generates, and it doesn't, so far as I know have regulator, as that part is usually in the lights), and I want to bring it down to 5V 500mA to power a gps that gets its power from a USB. So there's the project. I think I need some sort of diode to bring the voltage down, but I also need something to smooth it out, which would be some kind of cap, wouldn't it? Like I said, my grasp on all of this is way too loose, but I do hope to understand it better soon. Thanks much, and I'll try to provide any extra information that might be useful, if it's needed, but right now, that's about all I can think of.

 

Ok, that's fine. I kind of thought that there might be some way to use some capacitor set up that might allow for some kind of very short-term storage, and therefore regulation of the output. Of course that thought probably stems from my super-clueless status! Oh well, isn't ignorance bliss! Well, so I had thought about trying to charge a battery that I could later use to power the gps, but I guess I'm not sure what kind of a battery I might be looking for, and what would be involved in charging it.
On a side note, one of the things that made me think of doing this is a product that came out last year that allows one to charge their phone or gps from the generator hub. Here's a link: http://www.peterwhitecycles.com/ewerk.asp
I would, in an ideal and possibly non-existent world, like to make something like that that was unadjustable, and much smaller, which is why I started looking into this in the first place. With that unit, they do say that with some devices you need to use a battery in between the charging unit and the device being powered to smooth out the voltage.
The last thing that I would like to make clear, though it may be of no consequence in any case, is that I'm hoping to charge the gps, not power it. That is an eventually kind of thing though since the gps I have at the moment can't be charged and used simultaneously, which is kind of silly, but I guess they didn't forsee people like me....
Thanks so much for the replies! I really appreciate the help.
-brendon

 

Haha, well yeah, it's also pretty expensive, which is another reason I'd like to do it myself....
But I am on my bike a lot, but it's rare that I would have a need for something like the E-WERK, so the price is completely absurd in my case.