If you're visiting Franconia Notch State Park or White Mountains National Forest, then you have to take the time to experience the Flume Gorge.
Access to the Flume is via The Flume visitor Centre and costs about $40 for a family of four (can't remember the exact rate, so please check the website.) Even the visitor's centre is pretty neat to browse around.
The full loop walking/hiking trail takes about two hours with all your stops to take photos. You can shave a bit off this by taking the shuttle bus up the first 20 minutes of trail.
You'll be treated to a beautiful stream that runs over some small falls, neat rock formations, nice vegetation, then finally, the Flume Gorge. A boardwalk brings you through the gorge all the way up to Avalanche Falls -- the entire walk is amazing! It feels like you're in Jurassic Park!
There are areas for all ages, from the Bear Cave and Wolf's Den for kids (small caves they'll love to run/crawl through) to the magnificence of the falls and Pool for others, there's something for everyone's tastes.
Do no miss this attraction if in the area!