Things I have especially enjoyed in or near London:

The Globe Theater and the adjoining museum
The National Portrait Gallery
Kew Gardens
The British Museum
The Victoria & Albert Museum
Hampton Court Palace

Big disappointments:

Stonehenge – it's just a bunch of rocks, and you can't even get very close to them. Sure, the archeology and interpretation of it is fascinating, but you can't see that, can you? I'd much rather read a book about Stonehenge, or see a documentary about it, than visit it.

The changing of the guard at Buckingham Palace – way too crowded with tourists to get a good view. It might be better in the off-season.