Ikedayama Park in Gotanda is a bit difficult to find, but this park is absolutely gorgeous. Not crowded, beautiful plants, flowers, a large pond filled with carp and a traditional water well. Entry is free and there is a toilet available in the park. If you have a stop over around Shinagawa or have to switch trains at Gotanda station, this park is definitely worth a visit.
Ikedayama Park is about 15 minutes walk from Gotanda Station or the JR Meguro Station. Just follow the signs or use google maps.
Official site (in Japanese): http://www.city.shinagawa.tokyo.jp/hp/menu000000400/hpg000000344.htm