Mix colors. If you buy a red pepper and get some flower shape stamps, it can add some flavor and color. Pickled veggies also come in a variety of colors and can add color and flavor to a box.

Go with small servings of a wide variety of foods. Most foods are edible for a few bites while cold or at room temperature so you don't have to limit yourself to something that you could eat an entire serving of while it's cold. I'll make mini chicken burgers (find a chicken meatball recipe and squish/panfry them into patties 2 inches wide), save some chicken wings if I take some home in a box or made extra, and maybe a few bites of leftover dinner meat. Keep in mind that chilling food dulls the flavors so don't be afraid to drizzle some flavorful sauce (like tonkatsu sauce) over it.

Same for veggies, go for variety. I also like to use silicon cupcake cups to keep everything separate.

When it comes to rice, not all rices are good chilled or at room temperature, I use nishiki rice and toss it with a little bit of sushi vinegar before chilling it for my boxes since it is edible chilled. I have a little sauce bottle to bring soy sauce with me.

TL:DR; Pretty much what I'm trying to say is that if you prep 3 ingredients and put it in a bento box, it won't be very pretty and may not be a good eating experience. Use a little bit of a wide variety of ingredients to keep things interesting and fun.