The top three floors of Rafael Vinoly's so-called "Walkie-Talkie" building hold an enclosed public garden — three storeys of Mediterranean and South African planting under a curved glass roof — with two outdoor terraces and 360-degree views over the City of London. The building was given planning permission on condition that the upper floors be a public space, and admission is free if you book a slot online in advance.
The view is the best free view from height in central London — the Tower of London, Tower Bridge, the Shard across the river, the dome of St Paul's, and the cluster of nearby skyscrapers (the Gherkin, the Cheesegrater, the Scalpel) right at eye level. Time it for sunset and the City lights coming on for the best photographs.
There are bars and restaurants on the same level if you want a drink with the view, but they are not the point — most visitors just sit on the terrace benches or walk the planted terraces. The viewing balconies can be windy: hold on to hats.