hamburger bites- mini hamburgers, and try those william sonoma hand-pie molds to do holdable apple pies!

The United States has great regional food, you could do a menu of local flavors, or a sampler of different classic American foods. Some ideas: New York or Chicago style pizza, NY cheesecake, Philly Cheesesteaks, different styles of southern BBQ, Key Lime Pie, New England Clam Chowder, etc. Miniaturized versions would be adorable at a cocktail party.

I'm English, the things I enjoyed in the US that felt 'new' were:
meatloaf
lots of varieties of bagel
fat pancakes (as opposed to crepes)

things that really were new tastes:
grape jelly
jacima
the more outlandish breakfast cereals

Fried chicken or even better - fried turkey, peach cobbler, something with mashed potatoes, apple pie, iceburg lettuce salad...

mini sliders with caramelized onions, sweet potato and parsnip fries, pulled pork or BBQ ribs, spicy peanut mix, veggie kabobs, shrimp cocktail, strawberries with fresh whipped cream and pound cake, apple pie with vanilla ice cream

Think of specialty items from different cities or regions of the US: New York cheesecake, Philly steak sandwiches, Chicago hotdogs or pizza, Cincinnati chili, southern fried chicken, Idaho potatoes, Wisconsin cheese curds... umm Texas toast? This gets hard after a while.

I don't know if your mention of mac and cheese is just a random example or something you do plan on serving, but if the latter, I don't think it's a particularly American food. I always see it as a staple in Irish pubs.

If this site is trustworthy:
"It is said that macaroni ... has been cooked and served with cheese in Italian homes, inns and restaurants for over 500 years."

cornbread, apple pie, fried okra, dogs, chocolate chip cookies (my French friends love those because they dont sell chocolate chips in France)

Ooh ooh Denver omelette, Buffalo wings, baked Alaska, clam chowder (New England or Manhattan)...

Bison - can't get more American than that

Pigs in a blanket and/or cocktail weiners or mini meatballs in barbecue sauce. American AND 1950s! :)

I second the chocolate chip cookies motion.

You clearly need finger food, so I suggest raw veggies and bagel or potato chips with ranch dip (ranch dressing is rarely found outside the US). Chips and salsa, while pretty Southwestern, are also pretty American. Spinach dip is also quite American, I think.

Blueberries are apparently difficult to find in Europe, so a mini blueberry pie or some kind of blueberry cake could be in order.

And don't forget the ice cream. Despite the popularity of gelato overseas, ice cream is a pretty American invention. Maybe combine ice cream with the chocolate chip cookies and make ice cream sandwiches?

Good luck with the party! It sounds like it will be fun and people have a lot of good ideas for you to peruse.

I agree with BBQ, Buffalo wings, burgers. Also...shrimp & grits (SC), jambalaya (New Orleans), peel & eat shrimp (FL), corn dogs (street fair food), etc. I also read that nachos are completely American, and I know white cheese dip is a SE US phenomenon...

I think bbq or southern food is your best "straight-american" fare. Also corn. Fried pies.

American peanut butter is different than most found in the rest of the world. A pb&j with grape jelly is truly an american taste.
As is Jell-o. (jell-o shots are perfect for a party and completely american.)
And so are Rice Krispie treats.
Not very high brow, but trust me, people fry chicken in a lot of places. Marshmallows? Not so much.
For a dinner I'd have to go with standard hamburgers (sliders), corn on the cob (or popcorn as a snack), baked beans, watermelon, apple pie etc. as suggested above.

I second (or fourth) the apple pie call, but in two other ways: apple pie on a stick, or apple cobbler in ramekins or shot glasses.

You could also try doing regional foods from all parts of the country - cheesesteak sliders for philly, small bowls of clam chowder for boston, fried chicken drumsticks or biscuits and sausage gravy for the south, some ribs from the midwest, gumbo or etoufee for new orleans, a cheese plate for wisconsin/vermont/and maybe some monterrey jack for san fran, some rice-a-roni, and some fish tacos for the san diego/southwest area?

You could do little cheesecakes and apple cobbler/pies for dessert.

Serve long island iced teas and manhattans and california wines.

Oh this is too much fun!
Can I come?

I second pigs-n-blankets!
--fried apple pies
--a red, white and blue jello mold (red and blue jello shots too)
--Mini PB&J sandwiches
--White Castle hamburgers!

Root beer or root beer floats...a lot of non-Americans are mystified by root beer.

I also love the suggestions for regional foods/specialties. I vote for korean tacos!

You can go 2 directions -
1) Only serve foods that are green (or dyed green)

2) If you're looking for American food, I'd throw a BBQ, if you can eat it on the forth of July, it should be included. Corn, apple pie, burgers, watermelon, mac & cheese

On Macaroni and Cheese:

"If this site can be trusted: 'It is said that macaroni ... has been cooked and served with cheese in Italian homes, inns and restaurants for over 500 years.'"

That's as may be; but, the version that we know of as "macaroni and cheese" -- in which the noodles are cooked and then baked in a bechamel-based cheese sauce -- was invented by Thomas Jefferson's chef. So I'd say that that makes it pretty dang "American." The Italian version you refer to could mean a simpler dish, with macaroni served with shredded cheese on top. Or the cheese alongside. But the baked-in-a-cheese-sauce thing is what most American's "macaroni and cheese" brain defaults to.

Secondly: seconding the "look for regional specialties and go with them". I think trying to find a single uniform "American" cuisine is kind of a fools' errand, because we're just too big. But there are lots of local cuisines -- Cajun, Tex-Mex, Southern, Midwestern, New England...

Seems like it might be possible to decorate a roasted chicken or turkey to resemble the bald eagle that is depicted on the dollar bill.

I think of Chili, Corn Dogs, Barbecue sliders, Fried Chicken drumsticks (everybody's favorite part of the bird anyways) with Biscuits doused in gravy and the all Amercian Kraft Mac & Cheese (meaning nuclear orange).

Rice Krispy squares for dessert!

For a cocktail party, I'd go with retro hors d'oeuvres (think Mad Men style) that your parents might have served:

Pigs in a blanket
Deviled Eggs
Buffalo wings & celery w/blue cheese dressing
Lipton Onion Dip and chips
Triscuits with cheese slices
Stuffed mushrooms
Meatballs in a Sterno heated serving dish

Toll House Cookies
Ice cream sandwiches
Popsicles (invented in San Francisco)

Mini sized servings of regional food sound like a lot of work for a cocktail party.

Philly Cheesesteaks!!