DestinationCoupons offers a CitySightsDC All Loops Bus Tour Unlimited Hop-on/Hop-off Adult Ticket in Washington, D.C., for $39.20. (Click the red "Book Now Online button to order your tickets.) That's a savings of $20 and the lowest price we could find. This ticket includes access to the Monuments & Landmarks Loop, Washington National Cathedral & Georgetown Loop, and Arlington National Cemetery & Pentagon Loop, as well as a night tour of the monuments and National ferry cruise. This travel deal is valid for tours through December 31.
Note: DestinationCoupons also offers a Child's Ticket for $31.20. That's a savings of $18.