The post above is correct, the Churchill War Rooms are unforgettable and if you're a Beatles fan, the Abbey Road experience is great. And for Harry Potter fans, going to the studio is also worth the effort, but book with a group. The London Eye is also worth it, but you should book a fast-pass on-line.

Don't do the Hop-on Hop-off bus in London. The traffic is so bad, you'll spend most of your time sitting in traffic.

Google Maps is great as it will direct you to the closest bus or metro to get to your desired location.