4@ matched(all the same) 4-to-6 cell RC Li-Poly packs of 3,700-to-5,000mAh capacity wired in a 2s2p configuration with one 4-6 cell four slot charger should do the trick.
Just a simple wire harness in a bag with four plugs for the four little RC packs and plug them in and go and then when you get home unplug them from the wire harness and plug them into the four slots on the charger.
That's my first though. "C" rating on those packs will be more then enough, I don't think they even sell them anymore with less then a 15C rating. Light weight and powerful and should give him just about the duration he is looking for.
I'm using a very similar set-up only in a 1s3p configuration with HobbyKing Zippy house brand name 6-cell 3,700mAh packs on one of my hill climber only builds that uses a motor that runs at lower voltage levels then the hub motors he is looking to use. With a hub-motor built for 36-48V he is probably going to need at least a 2s configuration with at least 4-cell packs to give him at least 30-volts or so, don't think you would want to run on much lower voltage then that.