Bagels are sturdy and hold up far better than bread when stuffed into an overstuffed pack. You can toast them in in your frypan, and top them with almost anything. Here are some inspirations:
cream cheese and salmon (you can get it in a pouch in the tuna isle)
peanut butter and honey
nutella
butter and cinnamon sugar
melted cheese and tuna (again, opt for the pouch)
hummus (bring along the powdered kind and add water)
pizza toppings
re-fried beans and cheese (dehydrated beans are awesome!)