You could add the non-expiry option onto your tickets and save the extra two days for another holiday, providing of course that the non expiry option is cheaper than the price of a two day ticket.
We still have days left on 10 day non-expiring tickets that we bought in 2009 and we'll be using them again this year. We'll have to swop my DD's child ticket for an adult ticket as she's 10 years old this time and we bought the ticket when she was 8. This is the last year we'll be using them though, we'll have to buy new ones next time.