Would you like to save as your currency permanently or for this visit only?
Definitely grab an Oyster Card to ride London's Tube. Not only will it cost less per ride, but there's a daily cap. Meaning you can travel as much as you like, but will only be charged for a certain number of journeys, the rest are free.
Enjoyed all the (free) museums but looking for something more adrenaline-pumping? Ride whitewater rapids at the Lee Valley White Water Centre, built for the games in 2012.