I've found NLG35 in paper money from De Nederlandsche Bank, and FRF2.50 in coins from La République Française. Both of these currencies were yielded in favour of the Euro in 2002. At face value, they'd then have been worth €15.88 and €0.38 respectively -- which would convert into about £14 or $23 today.
So what can I do with it? Can randomly-chosen charities convert this old money into something useful?