Emergency suggestion just in case you don't find anything open (I don't live in London, sorry)
Treasure Hunt - make a list of things like red leaf, white feather, brown stone, snail shell etc, take them to a local park/wood/open space and give small prize for winner
Or you can make a list of things like yellow car, round window, wooden gate etc and take them for a long walk to find the things on the list... sounds awful, but mine LOVE it
We also do open-air winter picnics (local park) with flasks of soup/cocoa and blankets etc, they think it's hilarious.
I hope you find somewhere open, anyway!