Dried fruit is much sexier in the little individual serve boxes. They come in a variety of fruit combinations, so you might have dried apples in with the sultanas.

I use many of the ideas already suggested, but I also add an occasional rotation of plain potato chips (but only brands that contain nothing else except potatoes, oil, salt). It's not the maximum healthiest thing but the kids appreciate having it every so often.

In summer we freeze individual yoghurt pots, and on very hot days I send frozen fruit juice (the kids rip them open like icecups).

FWIW - I think lunch box "treats" have more to do with variety than whether it comes from a shop or not. I often send a container of savoury leftovers (eg pasta) or a small container of dip with things to dip into it. The kids love that kind of thing.