For short term use, say a minute or two a capacitor ought to do the trick with *perhaps* better efficiency than a battery (engineers please correct my theory here.)

For more utility I think the current crop of dynamos need more output. 12V @ 1A ought to be real useful. Imagine the uses...

To address concerns towards the drag of such a generator one must keep in mind our limited ability to generate power and how one wants to use it - for transportation, for electrical potential, for bragging rights, etc. Pick your own.