There are a huge number of trails to choose from in the forest. It is very mountainous and at night you can get snowstorms on the peaks(even in the summer). Watch out for wildlife and have a great time. There are not alot of people so be sure to bring the right gear.
How to get there:
From Manchester: Take Rt 93 north and it will end up in the forest. There are numorous towns in which you can stop and ask some locals about trails. The best way to do it is to just get some topo maps.