Just a couple of ideas;
Drinks Can Bowling
Set up a row of drinks cans in a line (or plastic bottles etc) along the ground. Put an elastic band between the wheels of the model, try to knock as many as possible over in one pass.
All up, last down.
Timed climb (say 10 seconds), last one down wins
Simple "band on" mechanism for dropping an "action man" with a chute, aim is to get him to land as close as possible to a mark on the ground. Score like archery, make a target on an old bed sheet.
slow pylon racing
Man on Man, mark out a "course" with broomsticks or similar. 5 laps (or however many), aim is to finish last, without cutting or turning round, must fly the course as it is laid out, so no looping or diverting.
Set out a trestle table, aim is to take off and land on the table (ie touch and go) as many times as possible in one minute (or any other fixed time). Penalties for missing a landing, hitting the table etc etc
Flat Spin Competition
Climb for a pre-determined time, then do as many flat spins as possible. Only flat spins score.
There are probably hundreds, just do whatever seems like fun, thats the name of the game after all