Narita used to have an observation deck back when it was just the North and South terminals which was where I spent my time when I had to spend 10 hrs there.

With an Admirals Club I think you can get into JAL or Cathay lounges which I think are better?

And some of this is going to depend on if you're motivated enough to visit Tokyo or Narita City. I think 10 hrs is enough to do either, but not sure if you'll have enough energy to do so. If not, there's lots of shops in Narita or you can hang at the lounges.