Greenwich is a good shout. The park is nice and there’s deer, observatory/planetarium, maritime museum, cutty sark, nice family friendly pubs about. Usually my go-to when the family want to come and visit.
Leadenhall Market, Time And Tide, London Mithraeum, Monument tower, Billingsgate Roman House & Baths, and the Roman Wall remains near Tower Hill tube are other nice bits near(ish) Sky Garden.