Having traveled extensively with babies in the past, I would suggest London as it has lots of high bridges which you can throw yourselves off of. 😜

But in fact your baby at 9 months is unlikely to be walking so that's in your favor as you will not be spending all of your time chasing after baby. With a baby in tow, I would be partial to the Netherlands and Belgium. Amsterdam is extremely child friendly and packed with children and it's walkable as long as you take care not to get run over by a bicycle. Trains from there to other nearby Dutch cities/towns plus Belgium. Bruges is charming, Ghent has the cool Gravenstein castle/fort. Easily accessible by train, you don't want to drive into these towns. There are some lovely small towns in Friesland -- e.g. Bolsward, Harlingen, Franeker, etc. -- where you can take a car. We drove from AMS to Franeker and used it as a base for day trips. Low key, charming, very, very child friendly. Smaller cities/towns are nice to be able to easily get to and from hotel for baby's nap or if baby just decides baby has had enough touring for the day.