We just got back from a week in London (mom, dad, two teenagers). Lots of good advice in this thread. A few other notes about London specifically:
- This has been alluded to, but London is expensive af right now, even with the strong(ish) dollar. As an example, a run of the mill sit-down restaurant family meal (including a beer or mixed drink for each parent) that would be $75-80 in the states often ran $125-130 in London.
- Can't remember when you're wanting to travel, but order a visitor Oystercard for London transport ASAP. These are great - they each come preloaded with 50 pounds of credit towards London tube and buses, at the least expensive rate, and you can top them off at any London Transport kiosk (in all Tube stations) as needed. We used the tube to get everywhere - it's very convenient and you never have to wait long for a train. Buses are handy in specific cases where you need to get somewhere that is inbetween tube stops. They get mailed to your house before your trip, so make sure you have enough time: https://tfl.gov.uk/travel-information/visiting-london/visitor-oyster-card
- Echo the strong rec for Yeoman Warder tour at Tower of London. I also recommend the Churchill War Rooms if you're a history or WWII buff, as you literally walk through the underground bunker where the British gov't kept running during the Blitz. We visited the Victoria & Albert museum as well, which was fantastic - it's not just art - it's furniture, textiles, historical exhibits - you name it.
- See a show if you can. There is a half-price tickets booth in Leicester Square where you can often get same-day tickets the morning of to various shows in the West End. We saw Back to the Future and Harry Potter and the Cursed Child (wife's pick) and both were great (we ordered both in advance).
- If anyone in your family is a Beatles/UK music fan, the Abbey Road Studios store is worth a visit - it's an easy walk from the St Johns Wood tube stop. It's cool enough to see even the exterior of such a famous studio (and the famous crosswalk outside) but the shop has all kinds of music-related shirts, vinyl, etc.
- We also did the GoCity Explorer Pass for attractions to save some $. The London Pass is an unlimited pass to a bunch of stuff - you pay by the day. But unless you're hitting up a ton of the attractions on their list, it's not worth the price. The GoCity Explorer Pass is different in that you pay for a specific number of attractions and then it's like a punch card - you just use them up. It got us discounted tickets to Tower of London, a hop-on-hop-off bus tour, Shakespeare's Globe and some other stuff. https://gocity.com/london/en-us/products/explorer