Having never had one, are onigiri filled with anything? I think of eating something that's basically entirely rice and wonder where's the protein? It seems to just be all simple carbs, which feels like a setup for an insulin spike and sugar crash.
You can make them all sorts of ways. Plain rice around a filling or with rice with stuff mixed in shaped into a ball.
You can also make a more sandwich like variant called onigirazu. This link is for one made with chicken katsu, but you can use any kind of sandwich filling--strongly flavored, savory things like ham work best.
They’re essentially like a dumpling.
If you’re looking for a healthier lunch option, try a 7 ingredient chopped salad including almonds and cashews and feta, topped with a simple vinaigrette. These take less than 10 minutes to make the night before, and will fill you up all afternoon.