Thames Clippers - hop on & off boats. Under 5s are free. We did it last year and will go again this year. One of the best ways to spend a day in London with kids - you hop off /off whenever.
Coffee/hot chocolate at Tower of London
Lunch at Greenwich (picnic)
Scooted around the O2
etc etc
DD just LOVED being on the boats (which are great, clean, spacious, run to a timetable and are buggy friendly) and has been asking all year to go again.
Picnics in parks.
Diana Playground is great on a non-sunny day (no queues).
Hyde Park - so much to explore.
South Bank has lots on for kids.
Battersea Park - amazing playground.
Holland park - wilderness area for tree climbing etc.
You can go the the Science Museum again - we live close and go there lots.
Does your 5yo scoot? Scoot along Thames with your and 18mo alongside.
Collect leaves twigs etc from park - next day give glue and cardboard and get them to make collages.
Do you meet up with DC friends in parks too?
Playgrounds with water play are the best - can keep DC entertained for HOURS!!
If you have a car have you been to Ruislip Lido?
If you have a BIG day out its good to have a quieter day to follow - library, art, home in garden etc
Another working parent who would totally LOVE 6 weeks off with the DC over the summer.