A couple of examples that caught my eye recently:
In Paternoster Square, the new-ish development behind St Paul's (which, incidentally, looks splendid now it's been thoroughly re-cleaned), is this solar calendar.
Clever, but is anyone likely to want to remember to be outside at noon on a sunny day to work out they're in the middle of June?
Just as clever, in the British Museum, is this lightbox re-evocation of an old Chinese painting of mountains and trees. It may look like a pen-and-ink wash, but it's done with the shadows of twigs, staw, pieces of paper and leaves (apologies for the wobbliness of this little video from my phone camera):
The Museum is full of surprises. Just at the moment, it has an Australian theme on, so the forecourt has a garden of Australian plants; once inside, this installation takes the visitor to China, and a small diversion finds (just at the moment) a display of ornate silver jewellery and brilliantly-coloured clothing from Oman, together with a similar display from the Balkans. And that's without spending any money to see, for example, the current special exhibition of treasures from Afghanistan. You could spend a lifetime travelling around the world without ever leaving Bloomsbury. Now that's ingenuity.