Issy's hands-down favorite song of the moment is "The People on the Bus". She can sing all the verses, including the penguins on the bus, the driver on the bus, and both the British and American versions of the mommies on the bus (chatter-chatter-chatter/yak-yak-yak). So I'd have to say that my entry for this week has got to be the Oyster Card, the magic little travel card that Transport for London introduced a couple of years ago:
It's a little slice of Harry Potter in your wallet, that you can slap onto special readers on the trains and buses, and your fare is automatically paid, taking you where you want to go. No more fumbling for tickets or tokens, no more piles of used transfers on the floors of stations, just total freedom of the TFL network. It's amazing.
Issy and I used ours today to ride the bus all the way to Chiswick, so we could buy some white eggs to dye for our Easter baskets. She entertained all the people on the bus with repeated performances of her favorite song. They love us on the E3.