How about a cook out? Pretty casual, dont you think?
With Alums and Actives only or (Alums with family and Actives).
Have at someones home, have people bring a dish, chips, etc.
One of our Alums does this @ 4 th of July Weekend. People bring fire works, has horse shoe pit, volley ball, or what ever comes to mind!
Usually @ 60-70 Members come and bring family to make a big thing that evryone can attend. Maybe, the first one you do should be members only and work up from there!
Give plenty of notice and have RSVP! Either ask for donations or have a core group go together to buy meat and buns, etc.
Good luck as we have a ball doing this! I always bring my Bachelor Dish=Chips