I've never asked my parents (or anyone else wanting to buy me a present) about giving a charitable donation instead: maybe greed on my part, or not being able to think of which charity would make best use of the money. Also don't want others to feel bad about wanting stuff for themselves by being the holier-than-thou charity gifter - or give rise to a 'but I wanted to buy something for you' situation.

Gift giving etiquette is so confusing. Of the above, I'd be tempted to ask for train fare money, but that doesn't really fulfil the definition of 'gift' as something square, wrappable and able to be put under the Christmas tree.