Well, there's a rather substantial Airshow going on
SCNR

Seriously, there is a program called "Singapore Stopover", which is for (paid) stopovers mainly for those on the Kangaroo route. I would imagine that they'll put you in one of the hotels that are part of the programme... and that you get their tour & sightseeing options as well. If not: I usually go to Chinatown just to walk around for a bit (after dark when it's a bit cooler), and I usually go to Sim Lim Square IT mall - although that is only advisable if you know what you need and how much it should cost.

Tom