It's a 20 minute, $20 Uber from the hotel to the heart of DC. The metro runs regularly and is less than a 10 minute walk from the Westin (closest metro stop in Alexandria is either Eisenhower or King Street Station). A metro ride to the center of DC would also take about 20 minutes and cost $2-3 per person. The water taxi is always a more scenic option.
The wedding party will be staying at the Westin-Alexandria and will offer reduced rates for people attending the wedding. These will need to be booked in advance and details are provided under "Travel."
If you're flying domestically or can get a connection, then flying to DCA/Reagan National is your best option. IAD/Dulles is the best airport to fly into internationally (40 minutes away) but BWI is only 45 minutes away so is still a good option
If you're just staying between Alexandria and DC then there's no need to rent a car - ubers, cabs, or the metro are all accessible.
If you're in a group then splitting an uber is the best option. If you're by yourself, then we recommend looking at the "Super Shuttle" which picks you up from the airport terminals and can take you anywhere. Super Shuttle's need to be reserved in advance and the price varies depending on when you get in/where you are going
New York is definitely recommended, especially if you've never been before, and want to be in the states a while. It's easy to get to from DC. There are more than 30 buses a day and it takes about 4 hours. You can also take the train (3/3.5 hours) or you could fly. Philadelphia is closer and just as easy to get to. Any other major city would require a flight but the 3 nearby airports make it easy to travel anywhere if there's somewhere that you really want to visit.
Averages between 14 degrees Celsius or 57 Fahrenheit to 16C/60F through the day, but can get down to 5C/41F at night in the first week in April.