Well the assault weapons ban is gone now so how about a nice M-16 with a high capacity magazine? Nothing says America more than the gift of firepower.
I think this too.
That combination in one place at the same time always ends very badly. Trust me on this one.
Hershey’s is now available in the UK. (And it’s nasty compared to British chocolate, IMO YMMV &c.)
They don’t have maple syrup in the U.K.? Man, it’s like the stuff grows on trees over here.
I’d second hyperjes’s book suggestion. Something nice and classy - maybe a pictorial history of your area, something like that. We’re not completely ignorant this side of the water, but there’s still huge swathes of America of which we have little idea of what they’re like.
And also bring a few hidously tasteless cheap tacky items, for a laugh. The sort of thing that you look at and think “who on earth buys anything like that?”
My friend in London always asks me to bring giant bottles of Aleve when I visit her. She also often puts in requests for music CDs, which are usually cheaper here.
Good maple syrup is a decent idea, as are books about your location. Political stuff will be meaningless in a couple of years. Excentric or extreme chilli sauces can be fun (ie those with rude labels, or Dave’s Insanity Sauce). Cinamon sweets like Hot Tomales aren’t common in UK. A jar of peanut butter and jelly (one jar containing both) would be of interest to Brits, but may never get eaten. Sports T-Shirts or Caps could be a good idea.
To epitimize fall in the North East, a book (and/or depending on when exactly you are leaving, a photo album of pictures of it) of fal foilage. And maple sugar candies. Probably even harder to get in the UK.
Something I learned from a similar thread here on the SDMB - when I visited a friend in London back in August, I brought a bunch of Reese’s peanut butter cups. Big Hit (and she normally isn’t that big on chocolate). I guess they’re not impossible to get over there, but very difficult. I’m sure the teenagers will enjoy them.
You’re from Jersey?
You’ve got to bring them some salt-water taffy or other Jersy shore stuff. Or how about some White Castles?
I once sent a British Doper pal (Francesca) a pro-2nd Amendment T-shirt. I was president of the Second Amendment Student Association at my law school at the time. The front had a circular logo with two 1911s and said, “Tech Law - Get Your Guns Up” (that’s one of the school’s sorta cheering mottos). The back had the text of the second amendment.
Can’t get much more American than that! And I felt quite sure that none of her friends would have a shirt quite like it.
(but yes, we do have it here - in any supermarket, it’s one of the things I must know i’ve got in stock)
Seconded, Reese’s peanut butter cups is what first came to mind when I saw the thread title (I’ve never seen them for sale here)
Billy the Bigmouth Bass.
Also, I bet saltwater taffy dipped in real maple syrup would taste gooooood.
I second that. Also, college or university related shirts and so on from your area. Things like that are light, easy to pack, non-breakable, and don’t look suspicious in luggage.
(For example, the University of Saskatchewan sweatshirts and shirts I brought from Saskatoon and gave to college age friends in the USA went over really well.)
I still wear it all the time. As a nightie You didn’t think I’d wear it in public, did you?
How about a cowboy hat or boots? I don’t know if it’s the same in England, but folks in France were crazy for American cowboy culture.
Can you get Girl Scout Cookies this time of year? They always go over pretty well for me.
Good thing you don’t. It would start with that, and then all you Brits will have guns again, and next thing you know King George would be back over here pushing us around.
You can buy them in many newsagents in London just in case you wanted some. Two definite examples in central London are the newsagents on the corner of Cheapside in the city (just opposite St Paul’s), and in the West End they are sold in a large general store/newsagent type thing along Davis St off Berkeley Square (it’s the only one there so you can’t miss it).
I only notice these things because I think they are disgusting. 
As you say, maple syrup can be bought in most supermarkets, but you can even get the real Canadian brands in the Canada shop just off Covent Garden.
I’d go with the scenic book of where you live. That’s what a Canadian brought my parents (showing the Rockies) and they loved it. I don’t know what you have in your area, but we don’t have many mountains and glacier lakes in London so that went down really well.
At their request, I took Canadians Thorton’s toffee and real Cadbury’s Dairy Milk chocolate. They just get crap made by Hershey’s disguised in a Cadbury’s wrapper over there and it tastes completely different. This is only my opinion, of course, but chocolate over the other side of the Atlantic all tastes like that really cheap stuff in novalty Easter eggs to me.
Good luck!