Hi Andy
Not my idea but why not try the Poker Fly Off Challenge (all day event)
Idea is to go to 5 local airfields and leave a packet of cards (with a responsible person) before the event day. On the day each pilot deposits a small amount into a kitty flies to each of the 5 airfields collecting 1 card from each pack returning to the home airfield to play their hand against other teams, best hand wins the kitty.
If the flying club charges a day rate for non-member passengers then this could be added to the kitty as well.
You can limit the airfields by giving a card out at the home field on departure and even reduce numbers further still by repeating this on return.
This can be flown by any number of pilots, you could also organise a lunch meet at the middle airfield, perhaps even negotiate free landings with the operator there!
Be good to hear other ideas!