Festivals can be held for any season or Holiday you like (such as fall, winter or Halloween) and offer a great way to raise a large amount of money for many different groups and clubs simultaneously.
First of all you'll need a space large enough to hold the Festival — consider a gymnasium, cafeteria or a suitable outdoor setting (avoid relying on the weather if you can). Let each of your groups run their own booth raise money in their preferred way.
There are plenty of games and activities to choose from: think of a Carnival Midway — face painting, caricature artists, toss the ball games, ducks in a pond or something like guess your weight are all useful ideas you might want to include.
Sell tickets to attendees which can then be traded at each booth, splitting up the proceeds with all of the different groups that are involved. A concession stand is a must, which can offer popcorn, hot dogs, cotton candy, lemonade and other treats to the people milling around.
To ensure a good turnout, put plenty of effort into advertising the event around the local area. This might involve flyers, posters and putting advertisements into the local press and media.
See this article for more summer fair related fundraising ideas.