The best activities for a farewell party are personal, not generic. Create a photo album or video with messages from colleagues who can't be there. Run a short quiz on the leaving person's career. Ask colleagues in advance to write a personal story or memory, then read them out.
Plan a memorable gift moment with thought for the person and their plans. And give the party an informal feel: the best farewells feel like a real celebration, not a compulsory programme.