Invite all the married couples onto the dance floor. If the minister is there, he/she can say a blessing over them; they can even renew their vows (a way to let them share in the theme of the day). After this, play a song for all the marrieds to dance at once.
During the song, start eliminating couples that were married less than 5 years, then less than 10 years, then 15… and so on… until the longest married couple remains on the dance floor. After the song is over (and their dance) have the newest married couple (whose reception this is) present the longest married couple with a boquet or some other token of recognition.
If done well, this is a sweet touch to any wedding reception.