I do not know if this is against your beliefs or not, but I had my graduation party last year and I could share a few ideas with you.
We rented a tent and tables from a local shop. We bought tablecloths at $1/pc from the Dollar Store and that worked out nicely.
For food, we had a couple large 5-gallon pitchers of Gatorade, water, etc. My father grilled meats (hot dogs, hamburgers) and served them as needed. We also had macaroni salad, fruit salad, and pasta. All of this was made at home. We decided against catering because it is very expensive, and the food is very high in calories.
Macaroni salad and fruit salad are good ideas though. Fruit salad is really just a mix of various fruits -- watermelon, cantaloupe, grapes, kiwi, whatever you like. Google will give you plenty of recipes for macaroni salad -- mayonnaise, peas, and boiled noodles.
Good luck with your graduation party! Tell us if you need any more advice!