Don't forget that this weekend is Open Garden Squares Weekend, when all sorts of London gardens and green spaces welcome visitors. One ticket (£12) gives you access to as many as you can visit in two days!
I enjoyed it so much last year that this year, I'll be volunteering at the Brunel Museum on Saturday morning. As well as a charming herb garden - with cocktails in the evening - you can visit the extraordinary Thames Tunnel shaft, now an atmospheric concert hall.
Another favourite of mine was Park Crescent, joined to Park Square by the Nursemaids' Tunnel. It also has cunningly-disguised ventilation shafts for the Underground station below, and some rather special plane trees.