Keiko and her apartment are fantastic!! We stayed here for 8 nights as a family of 4 with a baby & toddler. It was a huge apartment - especially for Tokyo standards - and really well equipped. It was opposite the Toy Museum which was brilliant with little ones (particularly as there is a play area for small children with outdoor toys right outside which was free!). The subway is a 5 minute walk away so we went all over Tokyo very easily. Shinjuku Gyoen is less than a 10 minute walk - we were lucky enough to be there during cherry blossom season - but it would be beautiful any season.
Keiko was an amazing host - helpful & generous! If I came back to Tokyo - I wouldn't stay anywhere else!!!