Class members may devise any number of appropriate activities. Planned around the appropriate holiday. Food and dress can be appropriate for the season. Various group and table games. Food could include anything from covered dish to snacks.
Guests can come dressed as they did during their high school years.
Bring pictures or high school yearbooks to share. Guests share things like the funniest thing that happened or something that went wrong at their wedding. Refreshments could include a wedding-type cake. Consider asking the pastor to lead a marriage renewal time. Play TV trivia-type games while eating popcorn and finger foots.