For my parents 40th wedding anniversary we gave the a chance to renew their wedding vows (they are Catholic). Then we had an afternoon tea at the church hall for all of the friends and family. For the immediate family, we then had a small sit down dinner in the evening.

For the tea, we had some of my mother's friends make all the goodies (and no sandwiches)

Good luck and I know that they will appreciate anything you do.