I'd say, first of all, that you shouldn't do the candy stages thing. The temperatures are very high, and the stuff sticks, so you're basically dealing with kids and napalm. Not fun from a safety perspective.
I like the flour thing, and it strikes me that you can play around with this by making basic tortillas: a flour-water paste, passably thick, that they roll out into tortillas. You can cook them in a toaster oven or a skillet or whatever, or bake them (check with somebody else for timing and temps), and then they can have tacos.
The thing is, you've got gluten to play with, and even some authentic historical reasons to do so: corn vs. wheat flour.
Get Quaker masa harina, ordinary wheat flour, and a few other products that don't cost too much but have significantly varying gluten content.
When the kids roll/pound the dough, they're going to see what gluten is really fast. With a fairly high-gluten paste, it will bind up and then start fighting back. Then you can refrigerate it covered for an hour and suddenly it will be soft and pliable again. With corn flour (masa harina), which has no gluten, it will never really bind well and certainly never start fighting.
When it's time to press out thin tortillas (which ought to be fun with corn: just put a big pot on it and have the fat kid stand in the pot), they'll see that gluten can be helpful, because the non-gluten tortillas will tend to break and the glutinous ones won't. This effect will be modified, but not basically changed, when the things are cooked.
Instead of tacos, you could make tortilla chips: just cut your tortillas in wedges, bake them with a spritz of spray cooking oil, sprinkle with salt when crisp, and you're good to go. Notice that the more glutinous flours produced tougher, bread-like chips, sort of like those pita chip things, and the non-glutinous ones were a lot more like tortilla chips.
The only thing is, explaining what gluten is and how it works is a passably complex matter in organic chemistry, so I don't know whether you can do this effectively on the lesson plan end. But maybe you'll think of a way?
Good luck!
Incidentally, here's a funny food trick that doesn't really match what you have in mind but might be cute some time.
Okay, so you're explaining about light refraction. Then you explain about internal refraction, where light can end up effectively trapped in a parabolic arc of a relatively dense transparent medium. So you set up a fish tank or something with a glass tap near the bottom, and you fill it with water. You line up a laser pointer so that it passes through the tank and out the tap, quite precisely. Turn on the laser, and see the point of light on the wall. Now open the tap and see the glowing red light trapped in the arc of falling water (which is quite beautiful, actually). Now explain that this is a very useful technique for studying light because it's trapped, and lift the beaker of water from the sink, still glowing red. Now drink it.
(Of course, you've put a beaker of red koolaid in the sink before you started, to make the trick happen. But if your students are up to this level, they'll stare at you, go "cooool," and then go, "hey, no way, you can't do that!" And you've taught a lesson and been cool, what more can you ask?)
