Westminster Abbey

http://www.westminster-abbey.org/

Until 6 or 7pm passing the gate (right of the church) you can enjoy a walk through the Westminster Abbey Choir School up to Tufton Street. 

Greenwich Park

A nice park with woods, playground and beautiful view on London city.


http://www.royalparks.gov.uk/Greenwich-Park.aspx

Leeds castle

Open all year around - One of the nicest castle around London

http://www.leeds-castle.com/

Allow about 3 hours to visit the castle, the gardens, the maze, the playgrounds with the kids, the bird space...
Better if it does not rain much as pretty much everything is outside (except 30/45min of visit within the castle itself)