Getting lost is an age old suggestion but a good one. Put away your cell phone or your A-Z and just wander around. I still do this regularly even though Ive lived here for nearly 10 years. London is a big city and it drip feeds you its secrets sometimes. You will come across things you wouldn't usually, in amongst the Starbucks and pret a mange there are some amazing sights. Just 'being' somewhere is a great experience and one no guide book can help you find.

THE place for birthday presents and bookcases (mine was 50p).

Favourite walk at night, especially in the summer.  Quite dramatic, very romantic. 

An obvious choice but no 'best of London' list is complete without the Barbican

I love this space.