Been there, done that fellow citizen!
If you're an aviation enthusiast, 5 hours are definitely not enough to consider leaving Changi for a second. I think you'll be using the non-SIA terminal and I'm more acquainted with the SIA terminal but I can tell you that there are enough things to see, eat and eventually shop for.
Like the others stated, it can take a 10 minutes ride to Changi and it can take a 3 hour one. In my own experience it took me 1 and a half hour once to cover the distance. That's mostly the case when you leave downtown at rush hour or shortly after.
I know they had plans to have the MRT (Mass Rapid Transit) operate all the way to Changi but I don't know if that's been achieved yet. If it is the case, then hop on it, go downtown, and find a food court! Use Google to find the MRT or Changi's website for more details.
Have a pleasant flight!
: Been to PER
, how is CNS