Hello magrett,

Have you checked this site? => http://www.changiairport.com/

One of my twice a year trips involves 5 hour transit at Changi (FUK/SIN/SYD) similar to your condition, and I can easily spend five hours without any problems. What I usually do is to catch a skytrain from T3 to T2 and walk back. You can drop by many gardens and etc.
I think Changi is very nice airport and you can do so many things, and you can find quite area (you can be almost by yourself). I think it is really up to you how you would like to spend.

Do not worry too much and just enjoy.