Whilst a surprise party is a lovely idea we have found over the years that by the time people are celebrating their golden wedding anniversary they don't like such surprises, and in any case, they probably want to be involved in the planning themselves.
The best answer seems to be to get all their close family and friends together and celebrate as one. The style of celebration is not what's important, and should be chosen to suit the couple themselves. Whether it's a house party in one of these large houses that you can rent these days. a dinner or party at a lovely hotel or restaurant close to where the couple live, or a lunch or afternoon tea, the really important thing is to make sure that family and friends are all there to celebrate with them.
50 years is a long time, and so there are hundreds of memories. One really special way to bring those memories to the fore, and to let the happy couple go away with something special is to prepare a memory book with photos and stories that are close to their hearts, and to get all the family, and friends to make contributions to the book - a sort of view of the golden couple from the perspective of each one of those that they love. You'll have them both in tears of course, but it really is quite magical