As JohnBuckner suggested, when the wind picks up (1:00pm) have a ground race (wings removed), we did this when winds gusted to 30+mph.

You could see who can be most fuel efficient and empty all the tanks and take a hypodermic needle and fill with 1 oz (or more) of fuel (electrics disqualified, ha) and time the flights, from takeoff to landing.

Have a prettiest/ugliest/lightest/heaviest competition for free soda/chips.

Egg drop. Attach a styrofoam cup with rubber bands and make a target circle on the field. Have the cup large enough so that the egg will not wedge in and refuse to come out. Requires enough experience to fly inverted, loop or roll.

Precision landing as with gliders. 25/50/100/50/25 point landing area. Can't loose any part of the airplane on landing (crash).