A big hit with the kids will be Kidzania (http://www.kidzania.com.sg/). Your child can spend all day there from opening until closing and probably not finish all the occupations! Only drawback is the entrance ticket is quite pricey. Another place I can think of is Tinkerfest at the Science Centre (http://www.science.edu.sg/events/Pages/Tinkerfest.aspx) Hope this helps!
Tinkerfest is fun. I run Saturday Kids, a digital literacy school for kids. We have a number of free tech and design workshops this Thurs to Sunday eg creating a GIF, making generative art using Scratch etc. https://explorer-pass.saturdaykids.com/sep-hols
Bowling and Kids water playground at Safra Punggol. Also, there are various courses that you can sign him for, he gets to interact at the same time. There 3D drawing course, clay modelling and creative cooking etc.
The beach or any of our national Parks are always a great idea
F Woon