Weather could be a factor, so could be your tolerance of crowds.
Been exploring since 1972. Never fails to provide rewarding experiences. Love the cemeteries, free museums, pub grub, meandering about the parks, church interiors, theatre, and riding the tube.
Some other lesser known sites:
The shopping area by the O2. Visually Interesting.
Mudchute Farm
The fallow deer in the vast Richmond Park and speaking of Richmond Park, the Isabella Plantation portion is lovely.
Barbican Conservatory is quite nice.
Coffee and breakfast at the former public urinal, The Attendant.
Columbia Flower Show
I do hope you get to Bletchley. Fascinating history.
Cafe is nice as well.