DC area:
When you’re in DC, don’t call it “Washington”. Nobody does that except for tourists and journalists. Call it “DC”.
If you happen to kill our president, someone will probably throw a party for you.
New York:
Don’t make eye contact on the subway.
People will be more confrontational with you there (IMbriefE) than in other parts of the US, in general. Service is worse in restaurants, too. Try not to let it get to you.
Try the pizza. A local place, not a chain like Pizza Hut or Domino’s. You may have to soak up some of the grease with a napkin before you can eat it.
Everything is going to be expensive, compared to the rest of the US. Thankfully, the dollar is pretty weak right now, so it probably won’t be a problem for you.
Both:
Use public transportation as much as possible. It’s there, it works, and it’s better than searching forever for a parking spot and then paying out the ass for it.
It will be chilly, but nowhere near as cold as what you’re used to, so pack accordingly.
US in general:
Don’t leave anything unlocked. Every single thing you own should be behind at least one lock or within your line of sight at all times. OTOH, people won’t freak out and call the Bomb Squad if you accidentally leave a purse or backpack lying around somewhere. If you do, though, get back to that place ASAP, and ask for “Lost and Found” at the place you left it, then call the police if anything is missing or you can’t find it.
If you have black or dark blue luggage, put something bright and distinctive on it so nobody mistakes it for theirs at the airport. Actually, that’s a good idea for all luggage, but black and dark blue especially, and especially at JFK.
Prices for gasoline and (in some places?) cigarettes will include all applicable taxes. Prices for EVERYTHING else will not, with few exceptions. Expect to spend anywhere from a few cents to a couple of dollars more than the listed price, because of this. Hopefully someone will chime in with the precise numbers for New York, DC and Maryland/Virginia. (If you have the time, there are lots of great historical sights (sic) in Maryland and Virginia that are fairly close to Washington, DC.)
Americans tend to be wary of people they don’t know looking at them, touching them or (to a lesser extent) talking to them. I have no idea how people are in Norway, but you should be prepared for that if it’s anything like the more touchy-feely European countries.
Take your driver’s license, passport or other ID everywhere. If you buy cigarettes or alcohol, expect to get asked for ID almost every time, no matter how old or young you look.
It’s considered polite to leave at least a 15% tip at a restaurant if you received decent service. Restaurant employees don’t get paid nearly enough to make a living here, so they rely on tips to put food on their own table. If you and your boyfriend have obviously non-American accents, the restaurant might automatically add 15% to your bill because they assume that Europeans don’t tip, so watch out for that. There’s nothing wrong with adding more if you really liked the food and/or the service.
When you do tip, try to use cash whenever possible, so the employees can decide whether or not to declare it to the IRS. Most restaurants leave a place for you to write your tip amount on your credit card bill; if you tip in cash, draw a line straight through that area. It usually says “Tip” or “Gratuity”.
Will you be here over Thanksgiving? It’s a big deal here. The whole holiday is all about intentionally eating WAY too much food. Most Americans of decent means realize how gluttonous that is, so they’re tickled pink to invite a foreigner or a needy person to their Thanksgiving dinner and share the wealth. It may or may not be rude to refuse seconds on any other day, but if you’re invited to Thanksgiving dinner, it’s definitely in poor taste to say “no” to any food that’s offered to you.