A river cruise is a great way to see the city and calm fractured teenage nerves. 12 pounds for an hour circle trip from Westminster, going under London Bridge, Blackfriars Bridge, and the Tower Bridge, taking in the Old Globe, Big Ben, the Shard, the Walkie Talkie, the Cheese Grater, and the Gherkin.
If you so choose, end at The Tower if you haven’t yet seen one the best sights in the city. The Tower is engrossing and could easily soak up 3-4 hours especially if you take the White Tower and Crown Jewels tours.
Pro tip: if pressed for time, take the one way trip from Westminster to St Katherine’s and alight at The Tower – there’s no sightseeing benefit in the reverse trip and there’s an underground station right behind the Tower. Also, do not underestimate how cold it can get on the water. The sort of “underground” souvenir shop just uphill from the Tower ticket office has ultracheap souvenirs, mittens, scarves.