For my parents I had a certificate type thing made that detailed every thing that happened on that day. It had what song was number one, what the price of a pint of milk , packet of cigarettes, average house was, A list of newspaper headlines from the day, loads of stuff really a bit like those day-you-were-born things. If you are quite creative you could find out this stuff and make up a certificate yourself, otherwise the scrapbook idea sounds really good so you could add them into there. I also got them a card from a more specialist staionary shop that had 25 years embrioidered on the front in silver thread with flowers etc, it was really pretty and I put it in a frame for them. Otherwise just go to a shop like Websters stationers, and buy 25 small things - silver charms, a little plant, silver aniversary keepsakes, a matching set of nice pens for them, silver keyrings, little teddy bear, bottle of champers, and put them all in a silver box with a silver balloon.