Loudoun County provides transportation to and from DC, for the price of $6/person each way via Loudoun County Transit. This could be an option. There is a pick-up point with plenty of parking right in Leesburg, off Catoctin Circle by Company 13's facilities (Loudoun County Rescue Squad).
Or, if you don't mind it taking a little while longer (about an extra 10 minute ride, if that) but want the trip to be a little more fun, drive up to Brunswick, MD (about 15-20 minutes west of Leesburg) and take the MARC train into Union Station from there. The price for this is $7/person each way (I think); you might be able to save a dollar or so by buying a round trip ticket. Again, plenty of free car parking is available for this trip.
I make the commute from just west of Leesburg every weekday and regularly use both of these options. They save gas, tolls, and prevent the stress of driving in what can often be one of the worst areas in the United States as far as traffic is concerned. Most of the people that use the train and the bus are commuting to and from work, but there's nothing that says people going for a day trip can't use these as well.
By the way, unless you're planning on parking and heading into DC after 10am, I'd recommend not planning to park at Metro facilities. During the week they fill up quickly-- there have been a number of times that I've circled the parking garages at some of these places for over a half hour and still haven't found a spot. Word of warning, too-- the parking spots at the Metro facilities that are reserved for permit holders until 10am are patrolled fairly well (at least they are at the Vienna station-- the closest Metro facility to Leesburg). It's a $40 ticket if you park in them before 10am without having one of the Metro's reserved parking passes.
Also, if you're planning to spend the day in DC I'd recommend buying an all-day Metro pass when you get there (if you plan to use Metro at all). It's valid after 9:30am and can be much cheaper than buying a regular fare card and using it to go in and out everywhere all day long.