Serious Eats - seriouseats.com

I've never made onigiri, but I'm gearing up for making bento boxes, so I've been contemplating onigiri. I've been wondering if it would work to make arancini onigiri, with cheese and maybe spinach in the middle. Especially if you're going to be able to nuke them, melting the cheese.

classics:
umeboshi (keeps well for long because of the anti-bacterial properties of umeboshi)
okaka (bonito flakes, wetted with soy sauce)
negimiso (scallions+ miso+sesame)
kombu (sweet-and-salty kelp, cooked down for this purpose)
tarako/mentaiko (raw or grilled cod roe (or spicy cod roe)
salted salmon (or salmon flakes that comes in a jar)

popular in the past couple of decades:
tuna-mayo (it doesn't keep well, naturally, but my fav)
cheese okaka (okaka with a new twist)
beef (thinly sliced beef, cooked down with mirin and soy sauce)

also, you can mix in any type of furikake. my favorite is yukari (plum-vinegar infused red shiso, dried). it makes cute purple/pink onigiri.
You could finely mince your zhacai to mix in to rice, too.

and I recently discovered Indian mango pickles... it makes an amazing Japanese-Indian fusion onigiri.

not traditional, but I like a nice green or kalamata olive in mine.

Umeboshi! Hands down my favorite. I have to say though that it's a flavor that people either love or hate. I can't grasp quite why anyone would hate it because it's so ridiculously delicious, but, there ya go.

I love chopped kimchi also, but it's really garlicky and maybe not something you'd feel comfortable eating and then rushing off to class after.

@deetroit is right on with the olives. They work really, really well.

The important thing to remember is that you don't have to use Asian ingredients. For a while there I was obsessed with making them with a mix of roasted jalapenos and cheddar cheese.

Spicy tuna is very popular filling, and deservedly so. Canned tuna, Kewpie mayo, and Sriracha.

Oh! And small cubed spam browned well, then mixed with wasabi. Wrapped with the nori it's like an awesome triangular musubi.

Some flavors are not going to work with the nori (like the jalapeno mix, or the olives) but others pretty much beg for it. If you want a wrapping that is not nori with accompanying ocean flavor, you can also use yuba, which is a wrapper made out out of tofu skin (it happens when you boil soy milk, much what happens with regular milk, but with far more delightful results). I know it may sound gross, but trust me, it's just wonderful.

I'm really interested to see what you come up with. Please let us know.