What can I take to lunch on a hiking trip?

Me and a couple other friends are going to this camp tomorrow, and this is my first time, we will go hiking, canoeing, do archery and camp activities. I have no clue what to bring for lunch, but it needs to be healthy, and keep me going, it's gonna be a long day! But I can't be too full, but I do have to stay strong, any suggestions on what to bring?