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.


Just the salt.  Side by side, on a dark plate with some small divider.  A pinch of each.  That's it.
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.
Fish.  Preferably raw.  A piece of sushi-grade salmon would be delicious.  Add a few flakes of salt.
Meat.  Take one very good, very fatty, very juicy piece of meat.  Sear it on all sides to about medium rare.  Salt.
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.
Fruit.  I recommend a piece of watermelon.  Possibly grilled.  With salt.
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...!