I agree. Lake flyer said, just let them know when you call for the cab and hopefully they will send you one of the cabs with the credit card machine in the back seat.

The Capitol is a cool tour. Make sure that they show you the old Supreme Court chamber. Speaking of which, I don't know if it is open for tours at the time you will be here but the Supreme Court building is right across the street from the capital so you may be able to do both in one day. If it isn't open, it is only a short walk down The Mall to either one of the Smithsonian museums or if you're up for a longer walk, the Washington Monument isn't too far and a nice walk down The Mall to get there.

If you do make it that far, all of the other monuments are just on the other side of the Washington Monument. One especially cool one to visit is the not very well known, but recently cleaned and refurbished DC WWI Memorial. It is a monument that many people even in DC don't know about, but very nice. It is on the left side of The Mall just past the Washington Monument. Another one I recommend that is less known is the Korean War Memorial which is further down and very impressive to visit when it is getting dark. Very eerie.

If you have a full day you can easily see most of The Mall area in a day.