I think, since most of the players are males, the rewards should mostly cater to masculine gender. As for the rewards, a box full of goodies containing condoms, a cock ring, a pocket pussy, probably a handcuff , and some erotic literature all custom made (or at-least bearing the logo of this game) would be a treasure box for us all.
You guys can even put it up as a promotion event where(lets say) if you buy X amount of kobans you would also get that box with it.
Anyways that's just my opinion and i just wrote what i would like to see in a goody box. Hope you guys like the idea