Like other moms, my st choice the park. I think babies like the "green view" & they'll be happy to see the other people/babies (and the sunshine is good to bb)!
Other options will be shopping mall (avoid peak hours), grandpa's home...etc. (wherever you have someone play with baby, they'll be happy). If you live in Shatin/Tsuen Wan, I think Snoopy Land in Shatin is also a good choice because of the eye-catching characters for babies!