Within Tokyo itself, best facilities I am aware of for a visiting 9 month old is the 0 to 2 section of the Tokyo Toy “Museum”
https://art-play.or.jp/ttm/en/
Not open on Thursdays.

Just outside Tokyo is the Tove Jansson Akebono Children’s Forest Park
https://www.city.hanno.lg.jp/section...kebono_en.html

An ideal place to picnic under the Sakura where a 9 month old (particularly one who is crawling and lifting themselves onto their feet) can be completely fulfilled and rewarded.

Tokyo is peppered with unexpected baby play areas, as parents it is vital that you look out for and recognise their boundaries so that nobody crosses them with their shoes still on. The boundaries can be as subtle as a change of colour or texture.
Prams/buggies also cannot cross; dirty wheels.