Near the grounds of the Imperial Palace, Kitanomaru Park is quite lovely to stroll through enjoying the changing of seasons reflected through the trees. The children however will be most impressed by the row boats available for hire in the Chidorigafuchi area.
Boating is only available from March 1 to December 15 and costs ¥500 for 30 minutes of ¥1,000 for 1 hour during most of the year except cherry blossom season when the price increases to ¥800 and ¥1,600 yen respectively. Hours are from 9:30am to 4:00pm (closed a little later during summer) and it is closed on Mondays. There are also several museums very close by such as the Science Foundation Museum and Tokyo National Museum of Modern Art.
1-1 Kitanomaru Koen, Chiyoda-ku
Train: 8 minutes walk from Kudanshita station (Tozai, Hanzomon, Toei Shinjuku lines); 2 minutes walk from Takebashi station (Tozai line).