If you know their friends or people who were IN your mom and dad's wedding, I would pick their brains for stories about how your parents met, what the wedding was like, any funny or sweet things they remember.
You could even do a little video of them telling the story and show it at the party, or just arrange for each person to get up and tell their memory or story of your mom and dad's courtship and wedding. If you have them tell stories AT the party, make sure someone in the family is recording it- that would make a lovely gift for your parents. But doing a video in advance might be better, in case anyone is not comfortable getting up in public to talk!
It might be a nice touch to play songs that were popular then or that your parents danced to in school or at their wedding. Have their best man and maid of honor get up to make toasts, as they would have at the wedding. Get all the old pictures you can from your parents or from relatives and make a big poster or computer slideshow of them. Did your mom have a favorite dress or color she liked to wear? Where did they go for their honeymoon and did anything happen there? ( my parent's got limburger cheese smeared on their radiator and it stank all the way there! Then my mom got the flu- but my dad bought her a gift that she still has, and made her feel better. Isn't that a good story???) things like that will make it a very personal, special event for them and all their friends and family. I am sure you will have a great party!!! Best of luck and congratulations to your mom and dad!!