What foods/preparations demonstrate the flavor difference between sea salt and table salt?

Much agreed with Krister that simple is best. Also, to really emphasize the difference, I'd aim for larger flakes of both the 'regular' salt and the sea-salt. Here are some serving ideas.

1. Just the salt. Side by side, on a dark plate with some small divider. A pinch of each. That's it.

2. Broth. A very simple vegetable broth, made with a little butter (unsalted of course) and fresh tomato, and perhaps a sprig of fresh thyme or rosemary, warmed to about 160 degrees, and strained through cheesecloth. I'd add about 2/3 the salt during cooking and a few 'fresh' flakes at service.

3. Fish. Preferably raw. A piece of sushi-grade salmon would be delicious. Add a few flakes of salt.

4. Meat. Take one very good, very fatty, very juicy piece of meat. Sear it on all sides to about medium rare. Salt.

5. Chocolate. Salt is a wonderful accompaniment to dark chocolate. And dark chocolate with caramel. Make or purchase such a confection, and serve two side by side with a few flakes of each salt on top.

6. Fruit. I recommend a piece of watermelon. Possibly grilled. With salt.

7. Tea. Find a good recipe for salt tea, popular in parts of Asia. Serve in two small Japanese style tea cups.

You have lots of options. Small portions will be key. Not overcooking anything will be key. Interesting options whether or not to tell your guests ahead of a dish which preparation is which, or let them to identify the difference with their palettes. Perhaps a combination, letting them try to guess at first, then with instruction, and finishing with guessing again. I'd add in some history, possibly from the highly recommended book Salt. Or just Wikipedia. Tell them all of the functions salt serves in our bodies, but especially in the transmissions of flavor on the tongue. Great idea...!

I think something very basic like a slice of ripe tomato or some avacado will help showcase the flavour difference between salts. Tomato would be my choice as it responds well to salting, really bringing out the tomato flavour. You can tell the difference between the 'harsh' salting of table salt vs the mellow flavour enhancement of something like Maldon, the king of salt.

Anything elaborate makes it hard to do comparisons. You could make your own butter with different types of salt and serve with some good bread. Another option could be drinks with a salted glass rim like margaritas.

Focaccia topped with salt.

Make real bavarian Brezes. I know that pretzel is used in English to mean basically any baked good which has the distinct shape, but in Bavaria, it is always made from soft yeast dough, pre-boiled in lye, covered with big salt flakes and baked. At home, it is acceptable to cook them in a strong baking soda solution instead of lye.

When you eat a bavarian Breze, you can distinctly taste the salt used on it. The dough softens/dilutes the salt concentration, so you don't have the sensory overload likely to be caused by eating a pinch of pure salt (which will keep you from noticing the fine notes of difference between the two salts), but it does not add to much of an own taste, so it doesn't mask the difference. Thus it should be very well suited for comparing the two salt types.

A recipe in English shouldn't be too hard to find. If you want an original German recipe, you can try this one. The translation rendered by Chromium is funny at some places, but generally well understandable, you shouldn't have trouble following it.

It is probably a good idea to use the different salt on separately baked batches, as the flakes get everywhere while baking and would mix if used on the same baking sheet.

When you have finished the taste test, you can eat the rest of the batch with cooked wieners and mustard (as per tradition), or just use it instead of bread.

Pure iodized salt is almost too salty whereas sea salt and other salts made with minerals are tempers it just a bit. Put a pinch a salad with a little olive oil (no vinegar). Sea salt enhances and complements the flavor trio but traditional table salt is just too strong and overwhelms. I enjoy the flavored salts---like smoked or Merlot salt.

Incidentally, salt on the salad is an old Roman tradition. Roman soldiers used to receive bags of salt as part of their payment. Hence the origin of the term "salary." Kinda cool!