Fort Rock and Newfields are ripe with trails. It's heavily mountain biked (the MTB community is currently building a lot of new trails on teh newfields side). Parking is located in a couple places on Rt. 85 on either side of Rt.101 (there's a large culvert under 101 connecting the two trail networks).

There's a nice network of trails off Bald Hill Road in Epping/Newmarket.
There's some nice trails in Great Bay National Wildlife Refuge near Pease in Newington with views of Great Bay.