Of course it would be fine to quit after the summer. Alot of the times fast food places are looking to hire a bunch of extra staff during the summer because they have so much more business!
I would suggest going up to the counter and just ask if they are hiring. Most likely they will say yes all are and give you an application! They fill it out either at home or at the resturant and give it back. They usually then give you a call and let you know if they would like to hire you or not!
Best of luck!