Sell Christmas Trees at Christmas time, Pumkins at Halloween time, Sell greeting cards, candy, merchandised products with the gym logo and/or other info, sell raffles, sell all kinds of stuff, t-shirts, etc.
Set up bake sales, hot dog stands, cotton candy days, and similar...
Organize flip flop clinics or recreational events that students pay extra for.
Gym and dine nights. Pizza parties. Fun at the beach days.
Carnivals, mini golf, Casino night, Bowling.
Sell photos, make a photo shot day and make some great pics to sell.
Celebrity night...invite famous gymnasts to meet.
Lectures, inspirational talks.
Pajama parties at the gym.
Dance competitions or other such gymnastics related events.
Start a bus service or car pool service for gym going students.
Keep donation jars in the gym and all over town.
Sell flowers for Mother's Day, Gift baskets. Do something for every holiday.
Run a flea market at the gym. Yard sale day.
Flip flop a thons and other pledge type events.