I would get bored spending 5 hours airside in NRT, even in a lounge, unless I had work to do.

There really isn't much on offer airside in NRT, IMHO, including restaurants. The choice of eateries landside is much wider (this is an option).

For 5 hours, I would consider a quick jaunt to Narita town, presuming you already have your onward boarding pass. This should give you about 2 hours in town.