If you're into history, you'll notice we have all these giant mill buildings along the Merrimack river. They used to be for manufacturing (at one time the longest contiguous mill buildings in the world), but now are used for hi-tech biz/manufacturing and education. If you want to learn more you can visit the Millyard Museum.

There's also the first Credit Union Museum (first in the US). The Currier Museum is also awesome as someone else suggested.

The Palace Theatre, along with it's mini-sister theater The Rex, is an historic theatre that's fun to see a show in (live music, comedians, etc at both with live plays/musicals at Palace).

Unfortunately it's been a bit too warm to send you to the ski/tubing slope or ice skating and too late in the season for Frank Lloyd Wright tours.

A walk around Livingston Park (Dorr's Pond) if you want a bit of exercise.

Strange Brew Tavern is a centrally located, relaxed bar that has live music often.

For amazing chocolate and other unique nibbles stop at the Dancing Lion on Elm St. If you need to bring home gifts for others, this is a great place to find it. They're on vacation until the 10th tho.