Although I appreciate it won't appeal to everyone, but if you're ever in London and enjoy perusing old documents, a visit to the National Archives at Kew is a must. Sometimes at archives you are made to feel quite unwelcome, especially if it's just a general visit of interest rather than one of specific research. This atmosphere doesn't exist at the National Archives and people seem glad to both help and see you.

Although this isn't a day-out as such, watching the amount of trains going through Clapham Junction is an interesting way to spend half-an-hour or so if the opportunity arises.