That might actually be a good idea, asking for a dimensional travel-capable hitchhiker's towel. Although I should probably just try to think of an appropriate price for some sort of cool gift, like the talent of erasure from Demons Don't Dream (one of Piers Anthony's Xanth novels, where the talent is a prize for winning the game Companions of Xanth).

On another subject, if someone were to wish for immortality (quite possible, seeing as Yuuko can create gods if she wants to), I would think that an appropriate price would be as such: The person can feel pain, and even lose their sanity for a period of time, but no matter what lengths they go to to die, or how sick they get of life, they are unable to leave the mortal plane permanently. Plus, they are cursed in such a way that events are always arranged to cause them trouble, and frequently at that. As an example, the person is inexplicably disintegrated. Said person must then spend decades dragging back and reassembling their atomized, very much scattered (by a convenient gust of wind) body, or remain as clusters of free-floating atoms.