funtrips, welcome to FT!
There's the SATS Premier lounge in Changi which you can pay a small charge to use. I haven't sat in the lounge but I have used the shower there. I seem to recall that you get a discount of some sort with your SQ boarding pass stub.
Have a look
here to see what's on offer at the airport (including, if your stopover is 5+ hours, a free city tour) - you can easily spend 5 hours there!