Although I use rice or bread in most of my bentos, I do like to mix it up with various other grains on occasion. Quinoa is probably my favorite alternate grain; it has a fun pop-y texture and nutty flavor, especially if you sauté it a bit in oil before steaming, and is so high in protein that it can considered to be a serious alternative protein source.
While most quinoa recipes seem to be vegetarian, this one is not, though you can easily turn it into a vegetarian or vegan dish. I've added just a little bit of dried sausage or saucisson sec though - its meaty, assertive flavor really goes well with the quinoa and the fresh peas. (In France, peas are often cooked with bacon.) Saucisson sec just means dried sausage, so you can use salami, chorizo, pepperoni, or any similar hard sausage that you can eat sliced without cooking. Whole brown mustard seeds add a little bite. This dish can be made in advance, eaten for dinner one day and bento a day or two later.
I've used fresh peas here, which are in season where I live, but frozen peas will work just as well.