This is a variation on the classic board game, Trivial Pursuit. For a reunion, you can do away with the formality of a board and play a free-form version.
Beforehand, the game organizer collects questions about the family and family lore. By quizzing family members, especially those with an interest in genealogy and family history, you can assemble a great store of interesting facts. Where did Grandpa serve during the war? What year were Uncle Pete and Aunt Milly married? What town in Poland was Great-grandpa Herman from? Which family member climbed Mount McKinley? Which branch of the family has the most kids in school? Who just graduated from Princeton?
Write the questions on 3-by-5-inch cards and put the answers on the back. Next, divide the questions into whatever categories make sense. You might have questions about "Ancestors," "Recent Events," "Love and Marriage" or "Wacky Relatives."
To play, divide into teams of two, three or four players. For each turn, a member of one team draws a card from a category and asks the question of the team whose turn it is. Move to a new category for each turn. Keep track of the number of right answers for each team to determine the winner.
The game is both fun and informative. Participants learn things they never knew about other family members and family history. Save the cards for future reunions and encourage relatives to add new ones.