If you just want to rest your feet for a while and just don't want to spent money at all for it. Benches and Chairs inside Changi Airport seems like a good idea, don't you think so? All you have to do is open your eyes widely, because it is there all over the airport.
But for a more comfortable chairs, I have to suggest the "Rest Area". You can rest for hours there sitting (and sleeping if you prefer) comfortably on a snooze chair, complete with head & leg rest and wake up timer.
For benches and chairs with a view, may I suggest a visit to the Gardens inside the airport (there're 6 of them) , the "Koi Pond" (Terminal 2), or to the "Children Playground" next to Mac Donald. For a man-made view, try the ones next to television (Departure/Transit Area).