This and the zoo are the two places you must go in Singapore. The conservatories are comfortably cool, beautiful, and educational. The whole park is an amazing attracting with minimal environmental footprint. This is my second visit when the Supertree walkway was closed, though - the first due to weather and second due to maintenance - so maybe call ahead if you have your heart set on that.
The park itself is free, but its worth paying to go into the conservatories.