Contact your congressman and/or senator. Tours of the capital are now escorted tours only and they will help you arrange a tour. Also, tours of the White House (very limited areas) require a ticket that you obtain from your congressman or senator.
The Washington Monument now requires reservations and tickets. You can reserve those online.
The Bureau of Engraving and Printing is a great tour. You actually see money being printed.
The great thing about DC is everything is available at no charge. Even venues that require tickets are controlling the crowd flow. Your only cost are hotels, meals, and transportation.
As for renting a car, rent one only if your leaving the DC area. There are no public parking places around town. They do have good public transportation, so you can take the train from the airport.
Wear comfortable shoes.
If you want more specific info, email me at rdgautney at yahoo dot com. Although I'd never want to live there, DC is on of my favorite cities to visit.