Its expensive, but a visit to the top of the Shard, the tallest building in Europe, on a clear day is wonderful.
360 degrees of London and its surrounds.
Alternative tip: If you want a view but not necessarily from the top 69th floor and you are dressed appropriately, (no shorts,slippers, sports shoes) make your way to the Aqua Shard Bar on the 31st for a cocktail and a view.