Quite a simple idea and the prizes can be rather small but why don't giffgaff organise a quiz night once a month (or even week)? Prizes could be something simple (a keyring or an IPhone5), but it would give other giffgaff members a chance to come onto the community, have a laugh and speak to people. Not only that but it might get them coming back to the community.
Obviously rules would apply (not using google etc) but it could be quite entertaining. One of the educators could even be quizmaster