I think the coolest SIN "attraction" is the T3 butterfly garden which has over 1000 butterflies.

SIN has a cinema in T2. A few foot reflexology as well as massage places (can get a full body scrub) and hair salons (manicure and pedicure avail.) spread over the 3 terminals.

The slide is at T3. As mentioned before there's a hawker centre in the basement of T3 (a food court serving various local fare).

There's a cactus garden and the aforementioned butterfly garden. Orchid Garden at T2 with a koi pond.

T2 also has an entertainment deck with a bunch of XBOX360s and PS3s with a few games.

There's a gym with showers.

The biggest gimmick at Changi is the T1 "social tree" which is basically a large tree shaped structure with a 360 degree monitor which has changing backgrounds (usually scenes and images from around Singapore). The "tree" is has about 8-10 screens at floor level where people can take photos with the forward facing cameras, write comments, throw in silly effects and frames and submit it to the tree where it will show on the 360 degree screen. You can also share the picture on Twitter and Facebook.