If you have access to a restaurant, hall or large event tent, holding a trivia competition is always a winner. People love to show off their knowledge of arcane topics and learn new factoids for future cocktail conversations. This one requires some planning, question prep, scoring savvy, and a master of ceremonies (or question and answer wrangler) with a great sense of fun. It also helps to have a public address system available.
We like the idea of having cooperative groups work together to answer questions and earn points for prizes. If the occasion involves school teams, clubs, couples or families, it's always fun for spectators to watch the interactions involved in coming up with group answers. Right or wrong, people watching at one of these events can be hilarious. To keep it civil, have each group appoint a team leader and spokesperson at the start of the game.
For a lively, fun evening, provide beverages and snacks. You'll also want Internet access and reference resources on hand to double check the answers and squelch any disputes.