You're probably going to miss some bits of the night safari, as the park doesn't open until 7.30pm and even by taxi, it's 30 mins back to the airport.

It's a long time since I was there (and I prefer the regular zoo to be honest), but I'm sure walking was better than using the tram. You can probably scratch the surface in that time, but....

So just how finely are you prepared to cut it? Changi is some size (as is the park) I'd want to be heading out by 10pm just to make sure there's no snafu in getting a cab, or a problem on the highway, etc etc.