Anything goes in a Bento, but all in small quantities so that your tastebuds don't get tired of the same foods. Variety, taste and appearance are the key-words.
Rice is an essential ingredient in Japanese Bento. Animal protein is optional, because rice, tofu and algae like Nori, Kombu and Wakame can be enough (they're really healthy and make great sources of vitamins and minerals). However, most Bentos contain raw or fried fish, eggs (sometimes raw!) and small chunks of meat or meatballs (though these are less common). Fruit is rarer in Japan than here, but it may be eaten at the beginning of a meal to whet the appetite.
Why so much variety?
The answer to this is Kuchi ga sabishii (literally my mouth is sad) if it doesn't get enough variety and taste! Your tastebuds should never get tired of the same food or flavour, and they should have a little bit of everything.
Everything in a Bento is cut into small portions that are easy to eat with fingers or chopsticks. Bento helps your palate get used to a variety of flavours and spares the boredom of chewing your way through endless pasta salads.