Pasir Ris Adventure Playground -Elias Road between carparks D&E.
Bit far if you're not on the East side but well worth a visit. It's divided into sections for different ages so there's something for toddlers to more grown up kids.
The wet/dry playground at the Bird Park is fantastic but only worth it if you have family membership and don't need to pay to get in otherwise it can be quite expensive to use as you have to pay normal entry fee.
These are my two faves (oh and my son's!)