Maybe an obvious choice, but the Barbican is always good. My favourite space in there is the curve gallery, last time I went was for Cory Arcangel's 'Beat the Champ'. I love the architecture, although not many people do. I think it was voted 'London's ugliest building' in 2003.
This is the place where ghost pirates roam in times forgotten. Where great poems were once written. Everything has happened here. A few abandoned boats and much stillness. And an island in the middle. Most curious. Not much choice but to explore as far as your nerves will take you. Watch out for Zorro the dog.
The Sackler Crossing is part of a route through the 120 hectares of Kew Gardens, designed by John Pawson in 2006. Set low to the surface of the lake, it seems to float across the water.