Activity: Big Ben at night. Tube Stop: Westminster. Cost: Free. Hours: Always open.
Once night falls, get on the tube to Westminster Station. Get out and BAM there's Big Ben. Hang out in the area for a little bit, and snap a picture with Big Ben at night. I liked crossing the footbridge and getting a the picture shown below across the street.
Find the red double decker bus and hop on Bus 11 going toward Great Winchester Street (east). Your Travel Card works on these as well as the tube. Sit in the top front overlooking the street and fly through the streets of London. I like to pretend I am Peter Pan. You'll pass Buckingham Palace and Trafalgar Square all lit up, ending at St. Paul's Cathedral. Go home and fall into bed. There's more of London to see tomorrow!