1. National Mall & Chinatown
Certainly the most notable, famous, and popular, this is the best place to start in my opinion. It has the highest concentration of tourist attractions and is also centrally located to get your bearings. Easy to get to by Metro from just about anywhere.
2. U Street
When I moved to D.C. this was definitely an up and coming area, with a block of establishments surrounded by some sketchy areas and people. It grew up a lot during my time there and is one of my favorite areas in D.C. Great food, nightlife, bars, and quirky shops. Nearby Columbia Heights is an extension of the vibe as well and was just getting going when I left. You can find jazz, electronic music, rock music, hip hop, cocktail bars, and good restaurants. It’s very easy to get to the main strip, just hop off at the U St metro stop and walk a couple blocks towards 14th St. You can also find some happenings towards 16th St. My best advice is to go late in the afternoon to wander a bit, then stay for dinner and drinks. Wander into any place that looks good.
3. Dupont Circle & Adams Morgan
If you’re really interested in the best, hottest, most popular nightlife, not to mention some of the best food, look no farther than Dupont Circle and Adams Morgan. These are two separate neighborhoods, technically within walking distance but it’s pretty far. But they’re close enough for an easy cab ride. Dupont is more of a collection of things, including shops, restaurants, bars, and nightclubs, namely along Connecticut towards M St.. One of my favorite hidden gems is the Mansion at O St located right in Dupont Circle. I would highly recommend this unique place Dupont is very easy to get to by Metro and has one of the longest escalators you’ll ever see. Adams Morgan is mostly bars and clubs, but restaurants and hookah are there as well. The National Zoo is nearby in Woodley Park if you happen to be there during the day, which I recall being free and quite nice. Not too big to take up your entire day. Adams Morgan is a good walk to the main strip across the bridge from the Metro stop, the main strip being along 18th St starting at Columbia Rd NW.
4. Eastern Market
If you’re into farmer’s markets this area of D.C. is for you. Fresh food market, restaurants, and people watching. Really nice area easy to get to by Metro. Belga Cafe is a good Belgium restaurant. Not too terribly far away is the National Arboretum which is one of my favorite nature places. It’s gorgeous, large, and easy to find some quiet and solitude. Capitol Hill is nearby which isn’t really worth much exploration, but if you have extra time and want to explore the small bar scene, it’s very local. A combination of politicians and local D.C. folk which can be kind of fun.
5. Rock Creek Park
My favorite nature destination in D.C. proper. Large park with winding roads, rivers, and quiet grassy spots if you look hard enough. I loved coming here to sit on the grass or near the river to just chill out, relax, and read.
Very wealthy area of D.C. also where Georgetown University is located. M St. is the main drag and there is also the waterfront. No Metro stops here unfortunately so you’ll need to cab or ride share. Plenty of expensive shops, restaurants, and nightlife.
I always considered the H St corridor in NE to be the younger brother of U St. It’s got a similar vibe, but less people know about it, or at least did at the time. The main event was the annual H St. Festival which was always great fun. Quirky events and drinking spots.
Possibly my favorite annual event in D.C. is embassy week, where almost all of the embassies from each country open their doors for all to visit and explore. They are mostly concentrated on embassy row, but some are located a bit farther away. There are quite a few, some of which are absolutely magnificent. I remember Indonesia, Peru, and the U.K. being my favorites. It’s all free, some serve food, some alcohol, and some have events. I went every year.
- Getting There. The two closes airports are Regan (domestic) located in Arlington just over the bridge and Dulles (international), about 30-45 minutes out in Northern Virginia. Washington Flyer used to service from the Dulles airport into the city, though there may be different options now. It was pretty direct and inexpensive as I remember it. Megabus is a great, inexpensive, semi-direct option if you're coming from New York, Baltimore, or Philadelphia. There may be other cities serviced as well.
- Getting Around. There is no need to rent a car. In fact you shouldn't, it will just be annoying to find parking. D.C. has one of the best public transportation systems in the country, specifically the Metro (subway). Not as good as London of course, but it goes all over and is very easy to use. Consider a Metro card if there for more than a couple days. The tunnels are very cool, and while the running joke is that there's always "track work" causing delays, just pay attention to the notices. At one point there was an app to see when the next train is expected for a given stop, tell you how to get where you're going, nearest metro stop, fares, etc. Not sure that still exists, but there's probably something. Buses are pretty good too for filling in the gaps where the Metro doesn't stop directly. They have also extended the Metro out to Northern Virginia, which I heard was expected to eventually connect to Dulles airport.
- City Layout. D.C. is organized into quadrants, each of which then have their own neighborhoods. You will often see streets suffixed with NW, NE, SW, and SE which indicates the quadrant. It's mostly a grid system with numbers and letters. State named streets are oriented diagonally and get confusing, so stick mainly to numbered and lettered streets to get around more easily.
- Accommodations. I lived in Arlington on the Virginia side of the D.C. area which I also thing can be convenient for visiting D.C. if you stay in a neighborhood like Clarendon or Rosslyn right by a Metro stop. If you want to stay in D.C. proper, maybe consider Chinatown or Eastern Market.
- Demonstrations. Depending on what's going on in the world, there may be demonstrations which will crowd certain areas, namely the National Mall up to the Capitol Building. This could affect public transportation as well as your ability to cut through crowds to various attractions.
- Events. Pay attention to sporting or other entertainment events, especially if multiple events are going on in a single day, as the metro will be packed and may delay where you're going.
- After Hours. One of the coolest things in D.C. is when they open up certain museums after hours for parties open to the public. I found them always to be fun. Sometimes for holidays like Halloween, or just some random weekend. You don't always get to see all of the museum, but usually some of it. There are always drinks and sometimes light food.