There are some amazing places open all over London for this weekend only, places which are normally closed to the public or which you have to pay to visit. All you need to enjoy a great weekend of architecture is:
a good map of London (I suggest picking up a walking map from an underground station)
a copy of the Open House London brochure (free from libraries or download from the website here
a good pair of walking shoes
a travel card or bus pass
a camera with lots of spare memory as you will surely take lots of photos
a willingness to practice the British art of queueing.
The guide lists over 750 buildings and events which are open in all the London boroughs. Some of the buildings and events are limited so check the brochure to see if you have to pre-book or when you need to turn up.
This event is an annual one - so if you miss out this year, you have to wait another 12 months for the next round.