There are several tour companies which operate daily. Theses range from monderately priced to expensive. There are several tours that leave from Union Station and tickets can be purchased at the desk at the main entrance.
The best way to find the tour that is right for you is to explore them by using a search engine and typing in "Washington, DC tours". There are many good companies that offer both day and night tours via bus, and river cruises as well.
If you are a photo nut, like me, I know one company even offers a tour with a proffessional photographer to help you get the best shots....it takes you to the area that will produce the best shot and the photographer helps you with camera settings and lighting to produce that perfect picture!
There are day passes available from automated machines at each Metro station the cost of a day pass ia approximately $6 US this is great if you are traveling between sites that are far apart (i.e...National Zoo and Capital Mall) it is not really worth the cost of a day pass if you are only going to one stop. A nice affordable way into and around town. If you ride stop and view Union Station its' main enterance is wonderful and outside sits the monument to Christopher Columbus. Tickets are available from their website. They also have a map of routes available.
Do not attempt to find a parking space in DC. It's nearly impossible. Find a hotel that is a short distace from DC and use park and ride with the Metro System. It's fast, clean and convenient.
Use the website that I have listed to find fares, locations, timetables and maps of the Metro system.
The Metro is perhaps the only public service that the District of Columbia runs well. It is well-kept, safe, and fairly accessible, covering most neighborhoods in the city. During the week the metro is open until roughly midnight, while closing at 3:00 am on the weekends to accomodate the clubbers and barhoppers..
One-way fair within the district is roughly $1.35, but can be more expensive if traveling to/from surrounding areas and suburbs.
I first used this Metro in 1979 when it was brand new and viewed as the solution to Washington's traffic problems. It is still clean although the ceiling looks like a loads of upside down sinks to me... what I found of immense value was not only was the Station Manager helpful and polite and there solely to offer assistance to dipsticks like me, but that HE HAD THE KEY to the bog.... not your usual half an hour wait while they find the key and you are crossing your legs...
Price of such a wonderful experience? $2.50 one way from Grosvenor Station (on Rockville Pike) directly into to the heart of Washington. It only took half an hour tops......
We took a Tourmobile (cost $20 each for the day), up to Arlington Cemetery (don’t eat or drink on the bus!) and all around town. This is great value for money. You just buy a ticket at the Kiosk near the Washington Monument and you can get on and off all day if you like. The last time you can board is 4.30pm. The guides are well informed and quite lively at times, not your usual drone of facts and figures.
Metro sucks! Especially if you are trying to get to Adams Morgan or Georgetown at night or anytime of the day for that matter...there is none. Disregard this if you are a professional working at capitol hill or Nova, you have ample train stations.