I'm growing cauliflower for the first time, and the results were kind of small compared to the rest of the plant. In searching whether the leaves and stalks of cauliflower were edible, I found this recipe: http://eatdrinkbetter.com/2009/05/31/got-more-leaves-than-cauliflower-ro... . I made it, and it was delicious, even at room temperature. The leaves were nice and crispy. I think it'd work well for bento.
When I made it, I didn't have any spring onions. I added broccoli and some Swiss chard to it, too, since I had a little of each.