I don't make obentou, but it depends on what kind you want to make. It can be as simple as rice with a dried plum on top, or it can have crab and octopus sausages with sweet fried egg and chicken and vegetables and of course, more rice, topped with maybe seaweed or sesame seeds.
Actually, you can us practically anything. I use sandwiches, but I turn them into something like pikachu and Rilakkuma. Anything and Everything can be put into your bento, it's just a lunch box of food. You can even put spaghetti in it. It doesn't have to be decorated or anything, but decoration is cute. The decoration and food shaped characters/things are made for a purpose to make little kids actually eat it, but if i don't like, even if it's cute, I still wouldn't eat it.
That's not healthy! lol You should remember your lunch so that you don't become too hungry. It's bad for your stomach to be hungry for too long. Skipping meals, or eating them too late, is also something that should be avoided.