Rice is a near-indispensible part of any meal in Japan. In fact, the word for cooked rice, gohan, is used as a synonym for 'food' or 'meal.' Bangohan means dinner, or literally, evening rice. So of course you'll find gohan in most bento boxes. Plain white rice, steamed in a rice cooker, is the baseline, but you may also find yummy fried rice. (Noodles are an alternative to rice, with chilled buckwheat noodles called soba being particularly popular in summer.) It may simply be packed in a layer of the box, or a compartment, or you may find it in the following form.
This is not really an item in itself but a sort of accessory to rice and onigiri. Furikake are savoury sprinkles which add flavour and colour to the rice, and are sprinkled over the top or mixed through. Onigiri may be rolled in furikake to give them a tasty coating. Popular furikake ingredients are dried flakes of edible seaweed, shrimp or salmon, or toasted sesame seeds. You can prepare your own or buy shaker-top jars or individual sachets in Japanese food stores. Furikake is also great on hot rice with any meal.
I don't mean just pickled gherkins/dills here (although if you like those, by all means put them in your bento), but all kinds of pickled vegetables and even fruit. Umeboshi, a pickled red plum with a tangy sour taste, is a classic, especially when served in the middle of a bed of white rice in imitation of the Japanese flag design. Again, try to find a Japanese food store in your area so you can sample the different kinds. You may get grossed out a few times but you may also find a new favourite, that's always my attitude to trying new food...
Although people are often surprised to hear it, the most common basic Japanese meal is curry (kare) and rice! The Japanese form of curry is based on the milder British adaptations of the original Indian dish, so while it's spicy, it's not the searing hot stuff they eat in Mumbai. It's a classic comfort food. It's also pretty close to liquid, so if you want to pack curry in a bento box, make sure it has a watertight seal, like a Tupperware lid, and include a spoon.
Of course, if you are really dedicated and virtuous you will get up early in the morning to cook everything freshly for bento. But if you are like most people you will often prepare an evening meal with an eye to including leftovers in tomorrow's packed lunches - cooking some extra chicken pieces, for example. To prevent monotony, they may be done up with some seasoning or trimmings to make them a little different. Or you may think, 'screw it, it's different enough that it's cold.' *^.^* Consult your own tastes and conscience! You may also like to make some favourite items ahead of time in a big batch (e.g. hardboil several eggs), and keep them in the fridge ready to use throughout the week.
Other Good Okazu
Chicken nuggets, croquettes of any kind, miniature/cocktail sausages, slices of teriyaki chicken or beef, meatballs, sushi rolls (maki), tofu puffs, sticks or chunks of cheese, chicken nibbles (wing joints), slices of cooked sausages, dim sum dumplings or pot-stickers (you can buy freezer packs in a Chinese supermarket, or indeed some mainstream supermarkets, and steam or fry them), miniature kebabs on cut-down bamboo skewers... You can also make sandwiches, yes, sandwiches, but with a cute bento twist... perhaps use mini-bagels as your bread (they can be cut in half to fit the box), or cut the sandwiches out using shaped cookie cutters. Circular crustless sandwiches are very bento. Try rolling the sandwich fillings up in a slice of bread, like a maki roll, and cut it into segments.
Browse recipe books, and use your imagination for more!