Good luck in DC! I just left not too long ago, but I really loved my time there.
The Metro is…not as good as it used to be, at least if you live far out. Fares have gone way up over the last several years, and track maintenance has become the norm on the weekends (at least it was when I lived in Vienna a couple years ago). Still, it’s a great way to get around inside the city, especially compared to driving. Get a SmarTrip card as soon as you can, as they charge a premium for using the disposable fare cards. They sell SmartTrips from automated kiosks in most stations these days.
I agree with the recommendations to fear the snow. All it takes is half an inch and the whole region loses its mind. It only takes 5-10% of drivers who don’t know what they’re doing in snow to screw things up for everyone, and in DC it’s probably closer to 30%.
Booze:
Montgomery County often has the lowest alcohol prices in MD, so you can shop local and usually be fine. Don’t buy booze in VA if you can avoid it, as they’re state-controlled and very pricey. Montgomery County is county-controlled, but they still tend to have very reasonable prices and frequent sales. They also have a searchable online database, so you can see which stores are stocking the items you want. When I lived in Herndon, I took occasional trips up to Bethesda to restock my bar.
If you’re into scotchy bourbon-style stuff, you may enjoy touring Copper Fox Distillery. They make some interesting whiskies using certain methods borrowed from scotch distilleries.
Check out the Dogfish Head Alehouse (Fairfax and Falls Church VA and Gaithersburg MD). Decent food and great beer. They’re based in DE, but I’d be very surprised if their offerings aren’t readily available on the West Coast. We get Stone and Lagunitas over here, after all.
PX in Old Town Alexandria. Expensive but unique cocktails. Old school speakeasy feel. Very fun for a fancy night out.
Food:
Astro Doughnut and Fried Chicken in Metro Center (they’re also opening a spot in Falls Church, VA).
Eamonn’s Dublin Chipper. Old Town Alexandria (same building and ownership as PX). Wonderful fish and chips. Also try their homemade sauces.
Eden Center in Falls Church, VA. Several excellent Vietnamese restaurants (as Acsenray already touched on).
Culture:
The Alamo Drafthouse in Ashburn, VA. It’s a bit of a hike from MD, so check out their calendar and see if they’re doing any events you’re interested in. I miss the Alamo so much. Good food, great tap list, comfy seats, and fun special events (think quote-a-longs and movie-themed feasts).
The Udvar-Hazy Airfield in Dulles, VA. They have a Space Shuttle, they have an SR-71 Blackbird, they have the Enola Gay, plus a gillion other amazing aircraft. Definitely worth an afternoon. Plus, afterwards, the Alamo Drafthouse and DFH Alehouse aren’t far away.