Wild Wild Wet, a water fun park. http://www.wildwildwet.com/index.aspx
you can go there by MRT and then walk for abt 10 mins. There are shuttle bus services on weekdays.
Botanical garden
There is a small water play in the children's area
(Jacob Ballas Children's Garden
at the Bukit Timah Core of the Singapore Botanic Gardens) http://www.sbg.org.sg/education/workshopchi.asp
you can go there by hopping on a Singapore city tour bus.
Singapore Zoo and night safari.
They are at the same place. Better take a taxi as the place is far away. If you take the MRT then change bus, it takes long long time to go there.