My DD who is now ten, really likes lots of different things she can pick at, rather than, say, a big slab of sandwich and a huge banana.
Also presentation is everything, so make it look appetising and interesting. The bento boxes are good as long as it is easy to get at all the food inside - so a single box with partitions, rather than a box with multiple layers that is difficult to get into.
My DD’s favourites:
Little cubes of cheese,
breadsticks,
Flavoured rice cakes (the ones for toddlers)
a very tiny pot of hummus
A small pot of salad, just big enough for 2 slices of cucumber, a cherry tomato and some thinly sliced red pepper, celery or carrot etc)
Little pots of either blueberries, clementine segments, a slice of watermelon, grapes, chunks of kiwi or tinned pineapple etc
A yoghurt squeezy
Sausage roll or cold sausage
Small sandwich or roll or wrap with filling
A pot of tuna sweetcorn Mayo, spoon, and half a roll
A nut-free biscuit
A fairy cake
A nut-free muesli bar
Crisps (I’d usually get a big bag and just put some in one of my pots, rather than send a whole bag of crisps which would get mostly wasted)
Friends kids also liked things like hard boiled egg, cold pasta salads, coleslaw, potato salad, couscous, etc but I could never interest my dd in anything too adventurous.
(I collect little pots from takeaways, sometimes you get really weeny pots with ready-made sushi kits etc, or plastic shot glasses with chocolate mousse in etc, if you save them you can build up a little collection of easy-to-open pots that you can fill with interesting things to munch on)