One thing that's happened quite recently (since the Frankfurt match in August) is that the Olympiastadion has introduced a cash free payment system for the kiosks around the stadium. The cards (one illustrated here) can be used to but snacks, beer, stadium punch etc from the many kiosks around the ground. |
carged back ointo the card when you take them back. You use it like an Oyster card for those familiar with that system.