First off, have a blast at Cedar Point! I'm fortunate to live near such a great park.
I just had this same experience when I took a girls' week with my mom, aunt, and godmother to the beach. I love to bake and sweets are like a drug for me. The ladies love my treats as well, and typically I'd bake a pan of homemade brownies, a batch of cookies, and so forth to not only eat in the long car ride but to snack on during the lazy day at the shore. Well, since I've adopted a healthier lifestyle and still have very little self-control when it comes to yummy chocolate items, I told the ladies that instead we'd snack on apples, nut bars, and similar items instead. They were so content with that! (It helps that each is in their mid-50s to early 60s and dealing with slower metabolisms.) Now, don't get me wrong: we ate awesome seafood dinners complete with bread/butter and full-fat sides each night, and splurged on ice cream nightly as well. However, we all agreed to spend our calories on some kick-butt meals that can only be had on the coast.
Be confident in your response to your friend and I'm sure you won't come across as rude.