The best thing about any charity shop is that you never know what you are going to find. I have a well tested route around London that includes all of my favourite charity shops. This Oxfam has a great selection of books and often turns up some interesting clothes too.
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.
I have just recently finished a project installing a drawing for artist Alan Johnston onto the ceiling of the Tate Britain. The experience, although being somewhat taxing on peoples health with long hours and difficult working conditions was fantastic. The mediative and mind altering drawing practice that Alan has created combined with the enthusiasm of a team of talented individuals has resulted in what can be considered a great work of art. It will be open to the public later this year.