For my dad's 60th, we had at least 60 people send him a card (we contacted about 100 to get that 60!), and had a small party. Everyone brought him 60 of something, and he got a real kick out of some of the little treats he got: golf balls, tees, gumballs, Mary Janes (he loved those!), socks, quarters, etc.
It may sound kinda small and silly, but he was like a kid at Christmas!
PS - let me know if you need me to send your dad a card!
