When my parents were married 25 years, my brother and I threw a party for them (OMG, this was ages ago...as they were married in 1962). Anyway, we got them a weekend away at the Concord Hotel. Now, my parents were both school teachers and never, ever went away anywhere, so this was perfect for them. But, you really have to know what your parents might like. If they are well-traveled then a weekend away won't be as special, I suppose. If they are like me, they might want something that reminds them of their children.