If you're in Singapore the Night Safari is a must!
It's fairly high in price but when you translate it to pound sterling and how much the London Zoo charges, it's much worth it.
If you really want to get a feel for all the animals at Singapore Zoo then do the full shabang and have breakfast with the orang-utans before returning in the evening for the Night Safari. The taxi is about S$12 from Orchard, but if you are in down town Singapore you might as well just jump onto the Zoo provided bus (details can be found on the website).
My time at the Night Safari with all the nocturnal animals was awesome but you might get really annoying people who don't know the meaning of NO FLASH PHOTOGRAPHY PLEASE! I've been to the Night Safari twice now the first time (2008) the tram was pretty empty and the ride was really fun. This time it got a bit frustrating at times.
The complimentary "creatures of the night show" is the highlight of my evening, but it's something I shouldn't talk of lest I mess up the surprise for everyone.