Shared helicopter flight
Helicopters don't come cheap, but you can save a few pennies by bunking up with some randos. Get airborne for either 12 or 18 minutes, which doesn't sound long, but when you can journey as the crow flies, you'll be surprised at just how much you can take in. Start in Battersea and fly down the Thames, past Green Park, Her Maj's Palace, The Shard, Big Ben and both Olympic and Wembley Stadiums.