Nashua and Portsmouth are great cities. Lots to do, good schools, low crime in Nashua and very low crime in Portsmouth. Both have areas that are a bit of a nature setting, though if you're looking for backwoods I'd look elsewhere. Portsmouth has the ocean, so it has a leg up over Nashua as far as that goes.

Concord is decent, if a bit dull. However, it's in very close proximity to backwoods and nature-type outdoor activities.

Manchester has some nice areas, but to be honest I think it's too expensive for what it is.

I'd also check out Exeter (next to Portsmouth) and Hollis, Amherst, or Milford (next to Nashua).