Depends what your interests are. Harrods is a good one to just have a wonder around as is Selfridges.

There is a Krispy Kreme in Harrods, and usually some pretty expensive vehicles parked outside/dropping people off.

Im a bit of a geek and LOVE going to wander around the British Museum, Natural History Museum, Science Museum....

There's the Trocadero which is good to kill an hour or so playing on the arcades...