Although these aren't in Nashua, they are an easy drive from there. These are some of my favorite gift stores:
Poles Apart in downtown Derry, which has many unique items, a little expensive but I can usually afford something around $20:
Poles Apart Artisan Gallery - Supporting the Handmade Artisan Movement
Also in Derry is Country Cupboard at 16 Crystal Avenue, as it says, lots of country-type items.
Whimsy Square in Amherst on route 101, in Salzburg Square, lots of fun stuff, especially for women:
Home Page Salzburg Square also has some other nice shops.
Crystal Orchid in Manchester, which is a florist, but has lots of nice gift items:
Crystal Orchid
I also like to shop along main street in Exeter, there is a store there with the NH League of Craftsmen items, and although some stuff is very expensive, it has a lot of affordable items too. There are also a few other nice gift shops there.
I'm sure I know more, but can't think of them right now.