You can make a ham and cheese sandwich (or any sandwich that your kid loves) and shape it like an animal head.
You can also use rice meals and shape into something that will catch the interest of your son/daughter.
You can also cut fruits in different shapes - square, star, triangle etc.
You check this link for other ideas:
http://www.yummy.ph/news-trends/baon-fun-6-bento-boxes-we-love?ref=feed_1