I’ll echo a lot of what people above have said. Water, plenty of water. Some sort of electrolyte drink is important too. But mostly water.
I like a PB&J sandwich in my bag for longer rides. Recently, I’ve added a few new potatoes, as a savory item to break up the sweetness. I do like the gels, but you’ve got to find one that your body likes. I recently bought a box of a new (to me) brand. First try, just didn’t feel good. Second try, well, let’s just say I’m NOT going for a third try. Fortunately, it was a Zwift ride, not out on the road. So it’s been back to my tried and true gel brand for me.
Whatever you do, don’t try anything new on the day. And listen to your body! And finally, don’t forget to have fun!! That’s something that gets lost sometimes, but it’s supposed to be fun.
Good luck, and have a great ride!!