Hey!
I know how you feel, it was my parents 25th the other year, and I didn't have any ideas. In the end me and my sister paid to get a painting done of the church where they were married, which was something special and related to them, but something they could keep as well. They had a party - invited lots of family who we don't normally see because they don't live near us and they got loads of different presents. I'll list what I can remember, it may give you some ideas:
Champagne, photo frames (got loads!!), a rose which was for a 'silver wedding' - has white flowers, vase, silver wine decanter thingy, wine glasses which had silver stems, (they were kind of silver globlet looking.)
The rose looked lovely, you can get ones sold for a silver anniversary from gardens centres, and its something they can keep and will flower each year. You could prehaps buy a (cheapish) bottle of champagne for the day and then something that they can keep. It also means that if you havn't got a present yet, in my case the painting wasn't reading for their anniversary they had got something (champagne) on the day and then had the painting later. Sorry I can't think of any more ideas! Hope you sort something!!