The things you know they'll eat... that might be something I can't really understand. That's an unhealthy sweet tooth from the looks of things, and I wouldn't go for that.
The celery or cukes (cucumbers)? Well, I'd do "ants on a log" by putting a nut butter (I use an almond butter of sorts) in the celery channel, and then adding raisins on it (I hadn't had raisins in a long time, though). For the cukes, I have no idea how that could work.
Granola bars and cookies, unless they're free of gluten, cow's milk, refined sugars, soy or seed oils (seed oils are hard to find missing from something), I'd stay the heck away from (there is a good chocolate raspberry granola from Seven Sundays I like, though I just treat that like a cereal).
Instead, I'd do the above mentioned things (minus the things they'll eat). For my school lunches back in the day, I had some sort of meat item, seaweed, a drink (usually a refined sugar-free juice box)
I'd do poop flavored chocolate, point blank. I've prepared for that somewhat with bean boozled in my past, though there are some "gross" flavors I legitimately liked (dead fish tasting like salmon for me, for example).