Are you going to build her a canoe as well ?

It seems to me that you intend on going to alot of trouble for one afternoon's worth of cycling for charity.

If it were my g/f, I would take 1/3 of the budget and buy a disposable MTB from a local big-box store and take the other 2/3 of the budget and donate the money to the charity or spend it on a post-event party.

Riding with her and snapping a photo, or otherwise documenting her effort would be a nice gift as well.

Good luck and have fun.

regards
Dan