Put on a Darts Tournament
As sports go, dart throwing requires a competitive spirit and a good throwing arm. Check with your favorite pub to see if it may be possible to rent a party room for the evening, and put on a darts tournament. If it's a weeknight, you may be able to negotiate a reduced rate. Charge admission and offer prizes. Guests will have to pay for their own drinks, of course, but the entertainment will probably be worth it. Businesses can enter competing departments, like the tech staff vs. the marketing department, and everyone will be able to relax for a few hours while donating to a good cause.